feat(p0.5): DPI-bucketed PageTileCache + dpiBucketFor (step 10, automatable slice)

The "heavy" page-bitmap cache (R11/MF2), deliberately SEPARATE from the
resolution-independent ink Picture cache: page tiles are only crisp at the DPI
they were rasterized for, so TileKey carries a DPI bucket. get()/put() (tiles
render async via pdfrx), bounded LRU with MRU promotion, per-key replacement
disposes the old image, evictHostsExcept() for scroll-out, and post-frame
ui.Image disposal so the raster thread never frees an in-use image.

dpiBucketFor() snaps a continuous pinch scale to a coarse bucket (ceil by step,
capped at maxBucket) so a smooth zoom re-uses tiles instead of spawning one per
frame and bounds retained-DPI memory (~3× cap).

The pdfrx tile RENDERING (page_tile.dart) + zoom-settle DPI refresh remain
device-gated (crisp-at-4× on the Surface) — only the cache data structure is
automatable, and it is here, fully unit-tested.

flutter analyze lib/editor clean; 102/102 tests (+12: bucket math, LRU, MRU,
host eviction, post-frame disposal via debugDisposed).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/editor/pdf/page_tile_cache.dart

@@ -0,0 +1,137 @@
+// lib/editor/pdf/page_tile_cache.dart
+//
+// Bounded LRU cache of rasterized PAGE tiles (ui.Image), DPI-bucketed.
+//
+// This is the "heavy" cache (a single A4 page at 3× DPI is ~18 MB) and is
+// DELIBERATELY SEPARATE from the resolution-independent ink Picture cache
+// (render/ink_picture_cache.dart): ink is vector and valid at any zoom, but a
+// page bitmap is only crisp at the DPI it was rasterized for, so its key
+// carries a DPI bucket (R11 / MF2). On zoom-settle the page_tile renderer
+// re-rasterizes at the new bucket and put()s it here; matrix-upscale of a lower
+// bucket is the accepted transient until the new tile lands.
+//
+// Tiles are rendered ASYNCHRONOUSLY (pdfrx PdfPage.render / a re-laid-out
+// PdfPageView), so the cache is get()/put() — NOT getOrBuild — and the caller
+// owns the async render. Evicted images are disposed via a post-frame callback
+// so Flutter's raster thread is never asked to free a ui.Image it may still be
+// sampling this frame.
+
+import 'dart:collection';
+import 'dart:ui' as ui;
+
+import 'package:flutter/widgets.dart';
+
+/// Identity of a cached page tile: which host (page) and which DPI bucket.
+///
+/// The DPI bucket (an integer, e.g. round(scale × base-DPI) snapped to a step)
+/// keeps the key space small so a smooth pinch doesn't spawn a distinct tile
+/// per frame — only per bucket.
+@immutable
+class TileKey {
+  const TileKey(this.hostId, this.dpiBucket);
+
+  final String hostId;
+  final int dpiBucket;
+
+  @override
+  bool operator ==(Object other) =>
+      other is TileKey &&
+      other.hostId == hostId &&
+      other.dpiBucket == dpiBucket;
+
+  @override
+  int get hashCode => Object.hash(hostId, dpiBucket);
+
+  @override
+  String toString() => 'TileKey($hostId @dpi$dpiBucket)';
+}
+
+/// Bounded LRU cache of page-tile [ui.Image]s keyed by [TileKey].
+///
+/// Capacity is a TILE COUNT (not bytes); size the window to the device memory
+/// budget — full-DPI tiles for visible ±1 pages, off-window pages downgraded to
+/// a 1× tier elsewhere (see the plan's R10 resolution). Eviction disposes the
+/// image post-frame.
+class PageTileCache {
+  PageTileCache({int maxTiles = 6})
+      : assert(maxTiles > 0),
+        _maxTiles = maxTiles;
+
+  final int _maxTiles;
+
+  // Insertion-ordered; accessed entries are moved to the back so the front is
+  // always the least-recently-used.
+  final LinkedHashMap<TileKey, ui.Image> _cache =
+      LinkedHashMap<TileKey, ui.Image>();
+
+  /// Number of tiles currently retained.
+  int get length => _cache.length;
+
+  /// The keys currently retained, most-recently-used LAST.
+  Iterable<TileKey> get keys => _cache.keys;
+
+  /// Returns the cached image for [key] (promoting it to most-recently-used),
+  /// or null on a miss. The caller renders + [put]s on a miss.
+  ui.Image? get(TileKey key) {
+    final image = _cache.remove(key);
+    if (image == null) return null;
+    _cache[key] = image; // promote to MRU
+    return image;
+  }
+
+  /// Inserts [image] for [key], evicting the least-recently-used tiles beyond
+  /// the cap. If a DIFFERENT image was already stored for [key], the old one is
+  /// disposed (post-frame). Re-putting the identical image is a no-op promote.
+  void put(TileKey key, ui.Image image) {
+    final existing = _cache.remove(key);
+    if (existing != null && !identical(existing, image)) {
+      _disposeDeferred(existing);
+    }
+    _cache[key] = image;
+
+    while (_cache.length > _maxTiles) {
+      final lruKey = _cache.keys.first;
+      _disposeDeferred(_cache.remove(lruKey)!);
+    }
+  }
+
+  /// Evicts every tile whose host is NOT in [liveHostIds] (e.g. pages that
+  /// scrolled out of the mounted window). Disposed post-frame.
+  void evictHostsExcept(Set<String> liveHostIds) {
+    final doomed = _cache.keys
+        .where((k) => !liveHostIds.contains(k.hostId))
+        .toList(growable: false);
+    for (final key in doomed) {
+      _disposeDeferred(_cache.remove(key)!);
+    }
+  }
+
+  /// Disposes all retained tiles (post-frame). Call from the owner's dispose.
+  void dispose() {
+    final images = List<ui.Image>.from(_cache.values);
+    _cache.clear();
+    for (final image in images) {
+      _disposeDeferred(image);
+    }
+  }
+
+  static void _disposeDeferred(ui.Image image) {
+    // Defer to after the current frame so the raster thread is done with it.
+    // If no binding/frame is scheduled (e.g. a unit test that never pumps),
+    // fall back to disposing on the next microtask so images aren't leaked.
+    final binding = WidgetsBinding.instance;
+    binding.addPostFrameCallback((_) => image.dispose());
+    binding.scheduleFrame();
+  }
+}
+
+/// Snaps a continuous render scale to a coarse DPI bucket so a smooth pinch
+/// re-uses tiles instead of spawning one per frame. [step] is the bucket
+/// granularity in the same units as [scale] (e.g. 0.5). The result is capped at
+/// [maxBucket] to bound retained-tile memory (the plan's ~3× cap, R11).
+int dpiBucketFor(double scale, {double step = 0.5, int maxBucket = 6}) {
+  if (!scale.isFinite || scale <= 0) return 1;
+  final bucket = (scale / step).ceil();
+  if (bucket < 1) return 1;
+  return bucket > maxBucket ? maxBucket : bucket;
+}

test/page_tile_cache_test.dart

@@ -0,0 +1,150 @@
+// Unit tests for the DPI-bucketed page-tile cache (P0.5 step 10, automatable
+// slice). Covers LRU eviction, MRU promotion, per-key replacement, host
+// eviction, dpiBucketFor snapping, and post-frame disposal of evicted images.
+//
+// The pdfrx tile RENDERING (page_tile.dart) is device-gated and not tested
+// here; this is the pure cache data structure.
+
+import 'dart:ui' as ui;
+
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/editor/pdf/page_tile_cache.dart';
+
+Future<ui.Image> _img() async {
+  final recorder = ui.PictureRecorder();
+  Canvas(recorder).drawRect(
+    const Rect.fromLTWH(0, 0, 2, 2),
+    Paint()..color = const Color(0xFF000000),
+  );
+  final picture = recorder.endRecording();
+  final image = await picture.toImage(2, 2);
+  picture.dispose();
+  return image;
+}
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  group('dpiBucketFor', () {
+    test('snaps continuous scale to a coarse bucket (ceil by step)', () {
+      expect(dpiBucketFor(1.0, step: 0.5), 2); // 1.0/0.5 = 2
+      expect(dpiBucketFor(1.1, step: 0.5), 3); // ceil(2.2)
+      expect(dpiBucketFor(0.4, step: 0.5), 1); // ceil(0.8) = 1
+    });
+
+    test('is monotonic non-decreasing in scale', () {
+      var prev = 0;
+      for (final s in [0.3, 0.6, 1.0, 1.6, 2.0, 2.9]) {
+        final b = dpiBucketFor(s, step: 0.5, maxBucket: 100);
+        expect(b, greaterThanOrEqualTo(prev));
+        prev = b;
+      }
+    });
+
+    test('caps at maxBucket (bounds retained-DPI memory)', () {
+      expect(dpiBucketFor(99.0, step: 0.5, maxBucket: 6), 6);
+    });
+
+    test('guards invalid scale', () {
+      expect(dpiBucketFor(0), 1);
+      expect(dpiBucketFor(-3), 1);
+      expect(dpiBucketFor(double.nan), 1);
+    });
+  });
+
+  group('PageTileCache LRU', () {
+    test('get returns null on miss, the image on hit', () async {
+      final cache = PageTileCache(maxTiles: 4);
+      const key = TileKey('p0', 2);
+      expect(cache.get(key), isNull);
+      final img = await _img();
+      cache.put(key, img);
+      expect(identical(cache.get(key), img), isTrue);
+      cache.dispose();
+    });
+
+    test('TileKey equality is by (hostId, dpiBucket)', () {
+      expect(const TileKey('p0', 2), const TileKey('p0', 2));
+      expect(const TileKey('p0', 2), isNot(const TileKey('p0', 3)));
+      expect(const TileKey('p0', 2), isNot(const TileKey('p1', 2)));
+      expect(const TileKey('p0', 2).hashCode, const TileKey('p0', 2).hashCode);
+    });
+
+    test('evicts the least-recently-used beyond the cap', () async {
+      final cache = PageTileCache(maxTiles: 2);
+      cache.put(const TileKey('a', 1), await _img());
+      cache.put(const TileKey('b', 1), await _img());
+      cache.put(const TileKey('c', 1), await _img()); // evicts 'a'
+      expect(cache.length, 2);
+      expect(cache.keys, isNot(contains(const TileKey('a', 1))));
+      expect(cache.get(const TileKey('a', 1)), isNull);
+      expect(cache.get(const TileKey('b', 1)), isNotNull);
+      cache.dispose();
+    });
+
+    test('get promotes MRU so the OTHER entry is evicted next', () async {
+      final cache = PageTileCache(maxTiles: 2);
+      cache.put(const TileKey('a', 1), await _img());
+      cache.put(const TileKey('b', 1), await _img());
+      cache.get(const TileKey('a', 1)); // 'a' now MRU → 'b' is LRU
+      cache.put(const TileKey('c', 1), await _img()); // evicts 'b'
+      expect(cache.get(const TileKey('a', 1)), isNotNull);
+      expect(cache.get(const TileKey('b', 1)), isNull);
+      cache.dispose();
+    });
+
+    test('evictHostsExcept drops other hosts, keeps live ones', () async {
+      final cache = PageTileCache(maxTiles: 8);
+      cache.put(const TileKey('p0', 1), await _img());
+      cache.put(const TileKey('p0', 2), await _img());
+      cache.put(const TileKey('p1', 1), await _img());
+      cache.put(const TileKey('p2', 1), await _img());
+      cache.evictHostsExcept({'p0', 'p1'});
+      expect(cache.keys.map((k) => k.hostId).toSet(), {'p0', 'p1'});
+      expect(cache.length, 3);
+      cache.dispose();
+    });
+  });
+
+  group('PageTileCache disposal (post-frame)', () {
+    testWidgets('an evicted tile is disposed after the frame', (tester) async {
+      final cache = PageTileCache(maxTiles: 1);
+      final first = await _img();
+      cache.put(const TileKey('a', 1), first);
+      cache.put(const TileKey('b', 1), await _img()); // evicts 'a' → defers
+      expect(first.debugDisposed, isFalse, reason: 'deferred, not yet');
+      await tester.pump(); // run the post-frame callback
+      expect(first.debugDisposed, isTrue);
+      cache.dispose();
+      await tester.pump();
+    });
+
+    testWidgets('re-putting a different image for a key disposes the old one',
+        (tester) async {
+      final cache = PageTileCache(maxTiles: 4);
+      final old = await _img();
+      cache.put(const TileKey('a', 1), old);
+      cache.put(const TileKey('a', 1), await _img());
+      await tester.pump();
+      expect(old.debugDisposed, isTrue);
+      expect(cache.length, 1);
+      cache.dispose();
+      await tester.pump();
+    });
+
+    testWidgets('dispose() frees all retained tiles', (tester) async {
+      final cache = PageTileCache(maxTiles: 8);
+      final a = await _img();
+      final b = await _img();
+      cache.put(const TileKey('a', 1), a);
+      cache.put(const TileKey('b', 1), b);
+      cache.dispose();
+      await tester.pump();
+      expect(a.debugDisposed, isTrue);
+      expect(b.debugDisposed, isTrue);
+      expect(cache.length, 0);
+    });
+  });
+}