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/system-overview.mmd

@@ -0,0 +1,21 @@
+graph LR
+    subgraph Watch["Pebble Time 2 (watch)"]
+        WatchC["Watch app (C)<br/>splash / main / wheel windows"]
+    end
+
+    subgraph Phone["Android phone"]
+        CFP["Core for Pebble<br/>(PebbleKit JS runtime, separate app)"]
+
+        subgraph Companion["Companion app process"]
+            HTTP["NanoHTTPD server :8888"]
+            UI["MainActivity / UI<br/>C64DisplayView, C64KeyboardView"]
+            JNI["JNI bridge<br/>vice_jni.c"]
+            VICE["VICE C64 core<br/>(own pthread)"]
+        end
+    end
+
+    WatchC <-->|Bluetooth AppMessage| CFP
+    CFP <-->|HTTP loopback 127.0.0.1:8888| HTTP
+    HTTP --> UI
+    UI --> JNI
+    JNI <--> VICE