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

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)

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

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

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

docs/diagrams/watch-wheel-input-flow.mmd

@@ -0,0 +1,15 @@
+sequenceDiagram
+    participant Watch
+    participant JS as PebbleKit JS
+    participant HTTP as NanoHTTPD /key
+    participant Main as MainActivity
+    participant JNI as vice_jni.c
+
+    Watch->>Watch: SELECT opens wheel, UP/DOWN rotate, SELECT confirms
+    Watch->>JS: AppMessage KEY_COMMAND = "LABEL"
+    JS->>HTTP: GET /key?cmd=LABEL
+    HTTP->>Main: onKey(cmd)
+    Main->>Main: watchKeyMap[cmd] → codes
+    Main->>JNI: injectKey(code, true) for each code
+    Main->>Main: postDelayed 80ms
+    Main->>JNI: injectKey(code, false) for each code

docs/diagrams/watch-window-stack.mmd

@@ -0,0 +1,5 @@
+stateDiagram-v2
+    [*] --> Splash
+    Splash --> Main: 1800ms timer\n(window_stack_remove splash)
+    Main --> Wheel: SELECT\n(push wheel)
+    Wheel --> Main: SELECT (send + pop)\nor BACK (cancel, pop)