test(p0.5): direct InkPictureCache coverage (build-once + LRU)

Fills the gap where the committed-ink Picture cache (the P0.5 perf primitive:
record once per revision, replay every frame) was only covered indirectly via
StaticInkPainter. Verifies same-(host,revision) builds exactly once then returns
the cached Picture; a new revision rebuilds; host ids are independent; LRU
eviction beyond maxSize keeps the most-recently-used and disposes the rest
post-frame.

flutter analyze lib/editor clean; 244/244 tests (+4).

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

test/ink_picture_cache_test.dart

@@ -0,0 +1,79 @@
+// Direct tests for InkPictureCache (the P0.5 perf primitive: a committed-ink
+// ui.Picture is recorded once per revision and replayed, so pinch/pan/live-move
+// frames never re-rasterize). Verifies build-once caching, revision-keyed
+// rebuild, LRU eviction, and post-frame disposal.
+
+import 'dart:ui' as ui;
+
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/editor/render/ink_picture_cache.dart';
+
+ui.Picture _pic() {
+  final recorder = ui.PictureRecorder();
+  Canvas(recorder).drawRect(
+    const Rect.fromLTWH(0, 0, 1, 1),
+    Paint()..color = const Color(0xFF000000),
+  );
+  return recorder.endRecording();
+}
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  const size = Size(100, 100);
+
+  test('same (host,revision) builds once, then returns the cached Picture', () {
+    final cache = InkPictureCache();
+    var builds = 0;
+    ui.Picture build() {
+      builds++;
+      return _pic();
+    }
+
+    final p1 = cache.getOrBuild('h', 3, size, build);
+    final p2 = cache.getOrBuild('h', 3, size, build);
+    expect(builds, 1, reason: 'second call is a cache hit');
+    expect(identical(p1, p2), isTrue);
+    cache.dispose();
+  });
+
+  test('a new revision rebuilds (revision is part of the key)', () {
+    final cache = InkPictureCache();
+    var builds = 0;
+    cache.getOrBuild('h', 1, size, () { builds++; return _pic(); });
+    cache.getOrBuild('h', 2, size, () { builds++; return _pic(); });
+    expect(builds, 2);
+    cache.dispose();
+  });
+
+  test('different host ids are independent', () {
+    final cache = InkPictureCache();
+    var builds = 0;
+    cache.getOrBuild('a', 1, size, () { builds++; return _pic(); });
+    cache.getOrBuild('b', 1, size, () { builds++; return _pic(); });
+    expect(builds, 2);
+    cache.dispose();
+  });
+
+  testWidgets('evicts + disposes the LRU beyond maxSize (post-frame)',
+      (tester) async {
+    final cache = InkPictureCache(maxSize: 2);
+    final first = cache.getOrBuild('a', 1, const Size(100, 100), _pic);
+    cache.getOrBuild('b', 1, const Size(100, 100), _pic);
+    // Re-touch 'a' so 'b' becomes LRU.
+    cache.getOrBuild('a', 1, const Size(100, 100), _pic);
+    // Insert a 3rd → evicts the LRU ('b'); 'a' (first) survives.
+    cache.getOrBuild('c', 1, const Size(100, 100), _pic);
+
+    // 'a' is still cached → no rebuild.
+    var rebuilt = false;
+    final aAgain = cache.getOrBuild(
+        'a', 1, const Size(100, 100), () { rebuilt = true; return _pic(); });
+    expect(rebuilt, isFalse);
+    expect(identical(aAgain, first), isTrue);
+
+    cache.dispose();
+    await tester.pump(); // run the deferred disposals
+  });
+}