Replace bundled ROM extraction with runtime import/download flow

Commodore ROMs are copyrighted and can no longer ship inside the APK.
MainActivity now detects missing ROMs on first launch and blocks
startup with a dialog offering two paths: pick files via the system
file picker, or download the official VICE 3.8 tarball and extract the
four ROMs from it client-side (minimal USTAR reader, no extra deps).

- Drop the extractViceRoms Gradle task and asset bundling; add
  res/extract_roms.sh for local sideloading during development instead
- gitignore keystores/keystore.properties ahead of a signed release
- Move architecture.md into docs/, refresh it for the screen-mirroring
  and watch-input additions, and add accompanying Mermaid diagrams
- Add docs/debugging.md and docs/publish.md (Play Store release notes,
  ROM-import compliance rationale)

docs/diagrams/watch-window-stack.mmd

@@ -0,0 +1,5 @@
+stateDiagram-v2
+    [*] --> Splash
+    Splash --> Main: 1800ms timer\n(window_stack_remove splash)
+    Main --> Wheel: SELECT\n(push wheel)
+    Wheel --> Main: SELECT (send + pop)\nor BACK (cancel, pop)