feat(p0.5): PageDocumentSource seam + fit-to-width metrics glue (step 10)

Connects the two pure P0.5 pieces: a minimal PageDocumentSource abstraction
(pageCount + pageSize, a pdfrx PdfDocument in production) and
pageStackMetricsForWidth() which fits every page to a single column width
(continuous-single) — height = columnWidth × aspect — feeding
PageStackMetrics.visibleRange. Defensive against non-positive page width.

This makes the windowing math consumable + unit-testable against a fake source
(no pdfium/GPU); the production pdfrx adapter is the thin device-side wrapper
added with the page-mounting widget.

flutter analyze lib/editor clean; 120/120 tests (+5: fit-to-width, gap, empty,
zero-width guard, windowing composition).

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

test/pdf_document_source_test.dart

@@ -0,0 +1,64 @@
+// Tests for the document→layout glue (P0.5 step 10, automatable slice):
+// fit-to-width metrics built from a fake PageDocumentSource. No pdfium.
+
+import 'dart:ui' show Size;
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/editor/layout/page_viewport.dart';
+import 'package:badnote/editor/pdf/pdf_document_source.dart';
+
+class _FakeSource implements PageDocumentSource {
+  _FakeSource(this._sizes);
+  final List<Size> _sizes;
+  @override
+  int get pageCount => _sizes.length;
+  @override
+  Size pageSize(int index) => _sizes[index];
+}
+
+void main() {
+  test('fit-to-width scales each page height by its aspect at the column width',
+      () {
+    // A4-ish portrait 600×800 and a square 500×500 at column width 300.
+    final source = _FakeSource([const Size(600, 800), const Size(500, 500)]);
+    final m = pageStackMetricsForWidth(source, 300);
+    expect(m.pageCount, 2);
+    expect(m.heightOf(0), closeTo(300 * 800 / 600, 1e-9)); // 400
+    expect(m.heightOf(1), closeTo(300.0, 1e-9)); // square → 300
+    expect(m.totalExtent, closeTo(700, 1e-9));
+  });
+
+  test('gap is threaded into the metrics', () {
+    final source = _FakeSource([const Size(100, 100), const Size(100, 100)]);
+    final m = pageStackMetricsForWidth(source, 100, gap: 25);
+    expect(m.offsetOf(1), closeTo(125, 1e-9)); // 100 + 25 gap
+  });
+
+  test('empty document yields empty metrics', () {
+    final m = pageStackMetricsForWidth(_FakeSource(const []), 300);
+    expect(m.pageCount, 0);
+    expect(m.totalExtent, 0);
+  });
+
+  test('non-positive page width is treated as zero height (no divide-by-zero)',
+      () {
+    final source = _FakeSource([const Size(0, 800), const Size(400, 200)]);
+    final m = pageStackMetricsForWidth(source, 300);
+    expect(m.heightOf(0), 0.0);
+    expect(m.heightOf(1), closeTo(150, 1e-9)); // 300 * 200/400
+  });
+
+  test('windowing composes on top of the fit-to-width metrics', () {
+    // 5 portrait pages 600×800 at width 300 → each 400 tall, total 2000.
+    final source =
+        _FakeSource(List.filled(5, const Size(600, 800)));
+    final m = pageStackMetricsForWidth(source, 300);
+    expect(m.totalExtent, closeTo(2000, 1e-9));
+    // viewport [350, 850) → page1 [400,800) and page2 [800,1200) just touched?
+    // page0 [0,400), page1 [400,800), page2 [800,1200). window [350,850]:
+    // first bottom>350 → page0 bottom 400>350 → 0; last top<850 → page2 top
+    // 800<850 → 2.
+    expect(m.visibleRange(350, 500), const PageWindow(0, 2));
+  });
+}