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-wheel-input-flow.mmd

@@ -0,0 +1,15 @@
+sequenceDiagram
+    participant Watch
+    participant JS as PebbleKit JS
+    participant HTTP as NanoHTTPD /key
+    participant Main as MainActivity
+    participant JNI as vice_jni.c
+
+    Watch->>Watch: SELECT opens wheel, UP/DOWN rotate, SELECT confirms
+    Watch->>JS: AppMessage KEY_COMMAND = "LABEL"
+    JS->>HTTP: GET /key?cmd=LABEL
+    HTTP->>Main: onKey(cmd)
+    Main->>Main: watchKeyMap[cmd] → codes
+    Main->>JNI: injectKey(code, true) for each code
+    Main->>Main: postDelayed 80ms
+    Main->>JNI: injectKey(code, false) for each code