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/screen-mirror-flow.mmd

@@ -0,0 +1,16 @@
+sequenceDiagram
+    participant VICE as VICE thread
+    participant JNI as vice_jni.c
+    participant HTTP as NanoHTTPD /screen
+    participant JS as PebbleKit JS
+    participant Watch as Watch app (KEY_SCREEN)
+
+    loop every 1s
+        JS->>HTTP: GET /screen
+        HTTP->>JNI: getScreenText()
+        JNI->>VICE: mem_read_screen($0400..$07E7)
+        JNI-->>HTTP: UTF-8 text (40x25, \n per row)
+        HTTP-->>JS: {"text": "..."}
+        JS->>Watch: AppMessage KEY_SCREEN
+        Watch->>Watch: update ScrollLayer/TextLayer
+    end