feat(p0.5): pdfrx-backed PageDocumentSource — static API source-pin (step 12/SF4)

Production adapter wrapping a pdfrx PdfDocument: snapshots page sizes via the
real pdfrx 2.4.4 geometry API (PdfDocument.pages, PdfPage.width/height) so the
pure layout math runs on the real document. Because it lives under lib/editor/,
`flutter analyze lib/editor` (the Oracle) type-checks it against the installed
pdfrx every run — a version bump that renames/retypes these members now FAILS
analysis instead of silently drifting (SF4 source-pin, statically).

Runtime contract + layout composition tested via the pdfium-free .fromSizes
ctor; the .fromDocument pin is the static guarantee (exercising it needs pdfium
= device path).

flutter analyze lib/editor clean; 123/123 tests (+3).

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

lib/editor/pdf/pdfrx_page_document_source.dart

@@ -0,0 +1,42 @@
+// lib/editor/pdf/pdfrx_page_document_source.dart
+//
+// Production [PageDocumentSource] backed by a pdfrx PdfDocument — the thin
+// device-side adapter that feeds real page geometry into the (pure, tested)
+// continuous-single layout math.
+//
+// SOURCE-PIN (plan SF4 / P0.5 step 12): this file references the exact pdfrx
+// 2.4.4 page-geometry API (`PdfDocument.pages`, `PdfPage.width`, `PdfPage.height`).
+// Because it lives under lib/editor/, the Oracle's `flutter analyze lib/editor`
+// type-checks it against the installed pdfrx on every run — so a version bump
+// that renames/retypes these members FAILS analysis instead of silently
+// drifting. (Static pin only: exercising it needs pdfium at runtime, which is
+// the device-gated path.)
+
+import 'dart:ui' show Size;
+
+import 'package:pdfrx/pdfrx.dart';
+
+import 'pdf_document_source.dart';
+
+/// Snapshots page sizes (PDF points) from an open pdfrx [PdfDocument] so the
+/// layout layer can read them synchronously (pdfrx loads pages asynchronously;
+/// once open, `document.pages[i].width/height` are available).
+class PdfrxPageDocumentSource implements PageDocumentSource {
+  const PdfrxPageDocumentSource.fromSizes(this._sizes);
+
+  /// Snapshot the intrinsic size of every page from an open [document].
+  factory PdfrxPageDocumentSource.fromDocument(PdfDocument document) {
+    final sizes = document.pages
+        .map((page) => Size(page.width, page.height))
+        .toList(growable: false);
+    return PdfrxPageDocumentSource.fromSizes(sizes);
+  }
+
+  final List<Size> _sizes;
+
+  @override
+  int get pageCount => _sizes.length;
+
+  @override
+  Size pageSize(int index) => _sizes[index];
+}