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/keyboard-input-flow.mmd

@@ -0,0 +1,11 @@
+sequenceDiagram
+    participant User
+    participant KB as C64KeyboardView
+    participant Main as MainActivity
+    participant JNI as vice_jni.c
+    participant VICE as VICE keyboard matrix
+
+    User->>KB: touch down/up on key
+    KB->>Main: onKeyEvent(key, pressed)
+    Main->>JNI: engine.injectKey(code, pressed)  (per code, for composites)
+    JNI->>VICE: keyboard_set_keyarr(row, col, pressed)