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>

lib/editor/pdf/pdf_document_source.dart

@@ -0,0 +1,45 @@
+// lib/editor/pdf/pdf_document_source.dart
+//
+// A minimal abstraction over a paginated document (a pdfrx PdfDocument in
+// production) consumed by the layout + render layer. Keeping the layout math
+// behind this seam lets continuous-single windowing (layout/page_viewport.dart)
+// and tiling be unit-tested against a fake source — no pdfium, no GPU, no real
+// PDF (the production pdfrx adapter is a thin device-side wrapper added with the
+// viewport widget / page_tile renderer, which are device-gated).
+
+import 'dart:ui' show Size;
+
+import '../layout/page_viewport.dart';
+
+/// Read-only page geometry for a paginated document.
+abstract class PageDocumentSource {
+  /// Number of pages (>= 0).
+  int get pageCount;
+
+  /// Intrinsic size of page [index] in PDF points (width/height > 0).
+  Size pageSize(int index);
+}
+
+/// Builds continuous-single stacking metrics by fitting every page to a single
+/// [columnWidth] (fit-to-width, the continuous-single mode): each page's
+/// laid-out height is `columnWidth × (pageHeight / pageWidth)`, preserving its
+/// aspect ratio. [gap] is inserted between pages (content units).
+///
+/// Pages reporting a non-positive width are treated as zero-height (defensive;
+/// real pages always have a positive width) so a malformed page can't throw.
+///
+/// Returns the metrics needed by [PageStackMetrics.visibleRange]; pair this with
+/// the device-gated page-mounting widget.
+PageStackMetrics pageStackMetricsForWidth(
+  PageDocumentSource source,
+  double columnWidth, {
+  double gap = 0.0,
+}) {
+  assert(columnWidth >= 0);
+  final heights = List<double>.generate(source.pageCount, (i) {
+    final size = source.pageSize(i);
+    if (size.width <= 0) return 0.0;
+    return columnWidth * (size.height / size.width);
+  });
+  return PageStackMetrics(pageHeights: heights, gap: gap);
+}