test(android): build out parity polish and device regression checklist

[markm39]

Why

Android native support is close enough that we need repeatable polish passes, not just ad hoc testing. This is a good contributor entry point because many checks are manual/device-focused and can be split into small PRs or reproductions.

Current state

The feat/android-parity branch has:

• Android drawing mostly working.
• Eraser responsiveness improved in manual testing.
• Android example run docs in example/README.md.
• Android Metro export and debug APK build passing locally.
• Known blocker: continuous pages past the second page still do not activate reliably for drawing. See bug(android): continuous pages past the second page do not activate for drawing #21.
• Known gap: Android benchmark runner/recorder is not implemented yet. See enhancement(android): port native benchmark runner and recorder #22.

Checklist to turn into repeatable coverage

Manual Android device/emulator checks:

• Draw with stylus and finger mode on page 1, 2, and 3+.
• Verify scrolling alone does not create extra pages.
• Verify exactly one trailing blank page appears after the last page with strokes.
• Verify dirty page capture survives scrolling away and back.
• Verify undo/redo after drawing, erasing, selection move, paste, and delete.
• Verify pixel eraser and object eraser responsiveness on small and dense pages.
• Verify selection handles, move, transform, copy, paste, and delete.
• Verify PDF background fit/centering and PNG/JPEG export scale.
• Verify orientation/size changes do not wipe the current drawing.

Automated coverage ideas:

• Add JS tests around trailing blank growth and dirty-page semantics for pooled reassignment.
• Add Android smoke checks where feasible for native module methods that do not require gesture input.
• Add a short documented manual QA script in example/README.md or docs/.

Done when

• The checklist above is documented and can be followed by contributors.
• Critical page growth / pool behavior has tests where practical.
• Any device-specific findings are split into focused bug issues with reproduction steps.
• Android parity status is clear from README/docs/issues without needing private context.