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>

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);
+    });
+  });
+}