feat(editor): M1 pdfrx spike + pen/touch capture

Add pdfrx 2.4.4. PenCaptureRegion routes stylus to ink (arena-bypass
via PenCaptureBinding) while touch falls through to pdfrx scroll/zoom.
Spike pane hosts PdfViewer with page-overlay ink at normalized coords +
frame-time HUD; reachable from home screen for on-device testing.
Bench/coordinate harness under integration_test. Generated assets are
gitignored (regenerate via tool/gen_*.dart).

pubspec.yaml

@@ -50,6 +50,7 @@ dependencies:
 
   # Embedded ONNX runtime (local OCR recognition backend)
   flutter_onnxruntime: ^1.8.0
+  pdfrx: ^2.4.4
 
 # Pin sqlite3 to the exact version whose native binaries are vendored under
 # vendor/sqlite3/ (see hooks block below). Without this, pub re-resolves to the
@@ -61,6 +62,9 @@ dependency_overrides:
 dev_dependencies:
   flutter_test:
     sdk: flutter
+  # M1 spike perf/coordinate gates (integration_test harness).
+  integration_test:
+    sdk: flutter
   flutter_lints: ^6.0.0
 
   # Code Generation