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)

CLAUDE.md

@@ -101,9 +101,13 @@ The NDK must be installed: **Android Studio → SDK Manager → SDK Tools → ND
 
 After the first successful build, subsequent builds skip the VICE compilation step entirely (outputs are up-to-date).
 
-**C64 ROM files** (`kernal`, `basic`, `chargen`, `1541`) all ship inside the VICE 3.8
-tarball at `res/vice-3.8.tar.gz` and are bundled in the APK under `app/src/main/assets/`.
-They are copied automatically to the external files dir on first launch — no user action needed.
+**C64 ROM files** (`kernal`, `basic`, `chargen`, `1541`) are copyrighted and are
+**not** bundled in the APK. On first launch, `MainActivity` checks the app's
+external files dir for these four files; if any are missing, it shows a blocking
+dialog that lets the user import their own legally-obtained ROM dump via the
+system file picker (see `docs/publish.md` §0). `res/extract_roms.sh` can pull
+the ROMs out of `res/vice-3.8.tar.gz` into `res/roms/` for local sideloading
+during development, but nothing in the Gradle build copies them into the APK.
 
 ### Key Kotlin/C files