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/snapshot-save-load-flow.mmd

@@ -0,0 +1,15 @@
+sequenceDiagram
+    participant UI as MainActivity (save/load button)
+    participant Pending as g_pending_save_state / g_pending_load_state
+    participant Refresh as video_canvas_refresh (VICE thread)
+    participant Trap as interrupt_maincpu_trigger_trap
+    participant CPU as 6510core.c DO_INTERRUPT(IK_TRAP)
+
+    UI->>Pending: saveState(path) / loadState(path)  [mutex-guarded]
+    Refresh->>Pending: drain pending path next frame
+    Refresh->>Trap: schedule save_state_trap / load_state_trap
+    CPU->>CPU: EXPORT_REGISTERS()
+    CPU->>Trap: run trap function
+    Trap->>Trap: machine_write_snapshot() / machine_read_snapshot()
+    CPU->>CPU: IMPORT_REGISTERS()
+    Note over CPU: reg_pc now matches the saved/restored maincpu_regs.pc