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>

test/page_viewport_test.dart

@@ -0,0 +1,107 @@
+// Unit tests for continuous-single windowing math (P0.5 step 10, automatable
+// slice). Pure geometry — no widgets, no pdfrx.
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/editor/layout/page_viewport.dart';
+
+void main() {
+  group('PageStackMetrics geometry', () {
+    test('cumulative tops and total extent (uniform pages + gap)', () {
+      final m = PageStackMetrics(pageHeights: [100, 100, 100], gap: 10);
+      expect(m.pageCount, 3);
+      expect(m.offsetOf(0), 0);
+      expect(m.offsetOf(1), 110); // 100 + 10 gap
+      expect(m.offsetOf(2), 220);
+      expect(m.totalExtent, 320); // 220 + last height 100, no trailing gap
+    });
+
+    test('variable page heights', () {
+      final m = PageStackMetrics(pageHeights: [50, 200, 75]);
+      expect(m.offsetOf(0), 0);
+      expect(m.offsetOf(1), 50);
+      expect(m.offsetOf(2), 250);
+      expect(m.totalExtent, 325);
+    });
+
+    test('empty document', () {
+      final m = PageStackMetrics(pageHeights: []);
+      expect(m.pageCount, 0);
+      expect(m.totalExtent, 0);
+      expect(m.visibleRange(0, 800), PageWindow.empty);
+    });
+  });
+
+  group('PageStackMetrics.visibleRange', () {
+    final m = PageStackMetrics(pageHeights: List.filled(10, 100)); // 0..1000
+
+    test('top of document shows the first pages', () {
+      // viewport [0,250) → pages 0,1,2 (page 2 is [200,300), overlaps top 250)
+      expect(m.visibleRange(0, 250), const PageWindow(0, 2));
+    });
+
+    test('mid scroll shows the intersecting band', () {
+      // viewport [420,720) → pages 4 [400,500) .. 7 [700,800)
+      expect(m.visibleRange(420, 300), const PageWindow(4, 7));
+    });
+
+    test('cacheExtent grows the window on both sides', () {
+      // base [420,720) → 4..7; +150 cache → [270,870) → pages 2..8
+      expect(
+        m.visibleRange(420, 300, cacheExtent: 150),
+        const PageWindow(2, 8),
+      );
+    });
+
+    test('exact page boundary is half-open (no spurious extra page)', () {
+      // viewport [0,200): page 0 [0,100), page 1 [100,200). Page 2 starts at
+      // 200 which is the exclusive bottom → NOT included.
+      expect(m.visibleRange(0, 200), const PageWindow(0, 1));
+    });
+
+    test('last page at the bottom', () {
+      expect(m.visibleRange(950, 50), const PageWindow(9, 9));
+    });
+
+    test('clamps to valid indices at the ends', () {
+      // Overscroll below 0 (cache reaches negative) still starts at page 0.
+      expect(m.visibleRange(0, 100, cacheExtent: 500).first, 0);
+      // Overscroll past the end still ends at the last page.
+      expect(m.visibleRange(900, 200, cacheExtent: 500).last, 9);
+    });
+
+    test('window entirely above content is empty', () {
+      // Scrolled far past the end with no cache reaching back.
+      expect(m.visibleRange(5000, 300), PageWindow.empty);
+    });
+
+    test('window entirely below content (negative, no overlap) is empty', () {
+      // viewport sitting above page 0 with bottom <= 0.
+      expect(m.visibleRange(-1000, 500), PageWindow.empty);
+    });
+
+    test('PageWindow helpers', () {
+      const w = PageWindow(2, 5);
+      expect(w.count, 4);
+      expect(w.contains(2), isTrue);
+      expect(w.contains(5), isTrue);
+      expect(w.contains(6), isFalse);
+      expect(PageWindow.empty.isEmpty, isTrue);
+      expect(PageWindow.empty.count, 0);
+    });
+  });
+
+  group('visibleRange with gaps', () {
+    test('gap bands are not page bands (a scroll inside a gap shows neighbors)',
+        () {
+      final m = PageStackMetrics(pageHeights: [100, 100, 100], gap: 20);
+      // page0 [0,100), gap [100,120), page1 [120,220), gap [220,240), page2 [240,340)
+      // viewport [105,115) sits inside the first gap → nearest pages 0 and 1
+      // both within a tiny window? window [105,115): page0 bottom 100 < 105 so
+      // page0 excluded; page1 top 120 > 115 so excluded → empty band in gap.
+      expect(m.visibleRange(105, 10), PageWindow.empty);
+      // A slightly taller viewport spanning the gap catches both.
+      expect(m.visibleRange(90, 40), const PageWindow(0, 1));
+    });
+  });
+}