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/companion-threads.mmd

@@ -0,0 +1,16 @@
+graph TD
+    Main["Main/UI thread<br/>Choreographer vsync loop, touch events"]
+    Vice["VICE thread<br/>main_program() → maincpu_mainloop()"]
+    Http["NanoHTTPD worker thread(s)<br/>one per request"]
+    StdoutT["stdout reader thread"]
+    StderrT["stderr reader thread"]
+    Audio["OpenSL ES callback thread"]
+
+    Main -->|"captureFrame(): reads g_framebuf"| Vice
+    Main -->|"injectKey(): writes keyboard matrix"| Vice
+    Http -->|"onKey → injectWatchKey → injectKey"| Vice
+    Http -->|"getScreenText(): reads C64 RAM"| Vice
+    Vice -->|"writes g_framebuf, drains pending queues"| Main
+    Vice -->|stdout/stderr pipes| StdoutT
+    Vice -->|stdout/stderr pipes| StderrT
+    Vice -->|sound ring buffer| Audio