feat(p0.5): continuous-single page windowing math (step 10, automatable slice)

PageStackMetrics + PageWindow: the pure geometry that decides which pages are
mounted for a scroll position (windowed lazy hosting → 60fps on a 300-page doc,
R1). Pages stack vertically with cumulative tops (O(log n) binary-search
visibleRange); a viewport [scroll, scroll+extent) grown by cacheExtent on each
side selects the inclusive intersecting page band, half-open at page boundaries,
clamped to valid indices, empty for empty/over-scrolled-past documents, and
gap-aware (a scroll resting inside an inter-page gap shows no page).

Widget-free + pdfrx-free by design: the page-mounting widget and zoom-settle DPI
refresh are device-gated; only the windowing math is automatable, and it is here
with exhaustive unit coverage (boundaries, cache band, clamping, gaps, empty).

flutter analyze lib/editor clean; 115/115 tests (+13).

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

lib/editor/layout/page_viewport.dart

@@ -0,0 +1,153 @@
+// lib/editor/layout/page_viewport.dart
+//
+// Continuous-single layout geometry: the PURE windowing math that decides which
+// pages are mounted for a given scroll position (P0.5 step 10). Pages stack
+// vertically; only the pages intersecting the viewport ± a cache band are
+// mounted (windowed lazy hosting → 60fps on a 300-page doc, R1).
+//
+// This file is intentionally widget-free and pdfrx-free: the actual page
+// mounting (a re-laid-out PdfPageView + the AnnotationLayer per page) and the
+// zoom-settle DPI refresh are device-gated and live in the viewport WIDGET +
+// pdf/page_tile.dart. The geometry is separated so it is unit-testable without
+// a GPU or a real document.
+
+import 'dart:math' as math;
+
+/// An inclusive range of page indices to mount. [isEmpty] when nothing
+/// intersects (e.g. an empty document or a scroll position past the end with no
+/// cache band reaching back).
+class PageWindow {
+  const PageWindow(this.first, this.last);
+
+  /// Sentinel empty window.
+  static const PageWindow empty = PageWindow(0, -1);
+
+  final int first;
+  final int last;
+
+  bool get isEmpty => last < first;
+  int get count => isEmpty ? 0 : (last - first + 1);
+
+  bool contains(int index) => !isEmpty && index >= first && index <= last;
+
+  @override
+  bool operator ==(Object other) =>
+      other is PageWindow && other.first == first && other.last == last;
+
+  @override
+  int get hashCode => Object.hash(first, last);
+
+  @override
+  String toString() => isEmpty ? 'PageWindow.empty' : 'PageWindow($first..$last)';
+}
+
+/// Vertical stacking metrics for continuous-single layout.
+///
+/// Page `i` occupies the half-open band `[offsetOf(i), offsetOf(i) + heightOf(i))`
+/// in content (scale-1) coordinates, with [gap] inserted between consecutive
+/// pages. Cumulative tops are precomputed so [visibleRange] is O(log n) per
+/// scroll frame.
+class PageStackMetrics {
+  PageStackMetrics({required List<double> pageHeights, this.gap = 0.0})
+      : assert(gap >= 0),
+        _heights = List<double>.unmodifiable(pageHeights),
+        _tops = _cumulativeTops(pageHeights, gap);
+
+  final List<double> _heights;
+
+  /// Top edge of each page in content coordinates (length == pageCount).
+  final List<double> _tops;
+
+  /// Gap between consecutive pages in content units.
+  final double gap;
+
+  int get pageCount => _heights.length;
+
+  /// Total scrollable content height (0 when there are no pages).
+  double get totalExtent {
+    if (_heights.isEmpty) return 0;
+    return _tops.last + _heights.last;
+  }
+
+  double heightOf(int index) => _heights[index];
+
+  /// Top edge (content coordinate) of page [index].
+  double offsetOf(int index) => _tops[index];
+
+  static List<double> _cumulativeTops(List<double> heights, double gap) {
+    final tops = List<double>.filled(heights.length, 0);
+    var acc = 0.0;
+    for (var i = 0; i < heights.length; i++) {
+      tops[i] = acc;
+      acc += heights[i] + gap;
+    }
+    return List<double>.unmodifiable(tops);
+  }
+
+  /// The inclusive page range intersecting the viewport
+  /// `[scrollOffset, scrollOffset + viewportExtent)` grown by [cacheExtent] on
+  /// each side. Pages whose band overlaps the grown window (even partially) are
+  /// included; the result is clamped to `[0, pageCount-1]`.
+  ///
+  /// Returns [PageWindow.empty] for an empty document or a window that does not
+  /// reach any page.
+  PageWindow visibleRange(
+    double scrollOffset,
+    double viewportExtent, {
+    double cacheExtent = 0.0,
+  }) {
+    if (_heights.isEmpty) return PageWindow.empty;
+    assert(viewportExtent >= 0);
+    assert(cacheExtent >= 0);
+
+    final double windowTop = scrollOffset - cacheExtent;
+    final double windowBottom = scrollOffset + viewportExtent + cacheExtent;
+
+    // No overlap with the content at all.
+    if (windowBottom <= 0 || windowTop >= totalExtent) {
+      return PageWindow.empty;
+    }
+
+    final int first = _firstIntersecting(windowTop);
+    final int last = _lastIntersecting(windowBottom);
+    if (last < first) return PageWindow.empty;
+    return PageWindow(first, last);
+  }
+
+  /// Lowest index whose band bottom is strictly after [y] (i.e. the first page
+  /// that the window's top edge does not sit fully below).
+  int _firstIntersecting(double y) {
+    // Find the first page whose bottom edge (top + height) > y.
+    var lo = 0;
+    var hi = _heights.length - 1;
+    var result = _heights.length - 1;
+    while (lo <= hi) {
+      final mid = (lo + hi) >> 1;
+      final bottom = _tops[mid] + _heights[mid];
+      if (bottom > y) {
+        result = mid;
+        hi = mid - 1;
+      } else {
+        lo = mid + 1;
+      }
+    }
+    return math.max(0, result);
+  }
+
+  /// Highest index whose top edge is strictly before [y].
+  int _lastIntersecting(double y) {
+    var lo = 0;
+    var hi = _heights.length - 1;
+    var result = 0;
+    while (lo <= hi) {
+      final mid = (lo + hi) >> 1;
+      if (_tops[mid] < y) {
+        result = mid;
+        lo = mid + 1;
+      } else {
+        hi = mid - 1;
+      }
+    }
+    return math.min(_heights.length - 1, result);
+  }
+}