feat: hot-swap LLM backend + media keys + codebase Q&A + scheduler cancel-by-text + TTS pre-warm

Single biggest user-facing change: you can now switch local↔cloud LLM by voice
without restarting. Plus four new packs and a perf nudge.

Shared LLM global (crates/jarvis-core/src/llm/mod.rs)
  - GLOBAL: Lazy<RwLock<Option<Arc<LlmClient>>>>. Single source of truth.
  - init_global() / current() / swap_to(LlmBackend) / current_backend_name() / parse_backend(str).
  - llm_fallback + llm_router migrated: they no longer own a separate LlmClient,
    they read from the global on each request. Hot-swap is now LIVE — change
    backend, next chat call uses the new one.
  - parse_backend accepts both english and russian aliases:
      "groq"/"cloud"/"облако"/"клауд"        → Groq
      "ollama"/"local"/"локал"/"локальный"  → Ollama

Voice + Lua LLM switcher (resources/commands/llm_switch/, 3 commands)
  - "переключись на локальный" / "перейди на оллама"  → swap to Ollama
  - "переключись на облако" / "используй грок"        → swap to Groq
  - "какой у тебя мозг" / "облако или локально"       → status query

  Underlying Lua API (registered globally):
    jarvis.llm({messages}, opts)   -- as before, now uses global client
    jarvis.llm_status()            -> "groq" | "ollama" | "none"
    jarvis.llm_switch(name)        -> name on success, nil on failure

Media keys pack (resources/commands/media_keys/, 4 commands)
  - "пауза" / "плей"           → VK_MEDIA_PLAY_PAUSE
  - "следующий трек"           → VK_MEDIA_NEXT_TRACK
  - "предыдущий трек"          → VK_MEDIA_PREV_TRACK
  - "стоп музыка"              → VK_MEDIA_STOP
  - Helper _media.ps1 uses user32.keybd_event (P/Invoke through Add-Type).
  - Works with anything that listens to global media keys: Spotify, Yandex Music,
    YouTube (focused tab), Foobar2000, Winamp, etc. No OAuth, no API keys.

Codebase Q&A pack (resources/commands/codebase_qa/, 3 commands)
  - "укажи проект <path>"             → stores path in jarvis.memory under codebase.root
  - "какой проект сейчас"             → speaks current path
  - "что делает функция X" / "найди в коде Y" / "объясни код"
    → walks the folder (depth 3, 30 files cap, 4KB per file, 50KB total),
      filters by source extensions (rs/py/ts/lua/go/...), feeds digest to LLM
      with a "senior reviewer" system prompt, speaks 3-5 sentence answer.

Scheduler cancel-by-text (crates/jarvis-core/src/scheduler.rs)
  - new pub fn find_by_text(query, limit) -> Vec<ScheduledTask>
  - new pub fn remove_by_text(query) -> usize (count removed)
  - new pure helper find_by_text_in(tasks, query, limit) for tests
  - Lua: jarvis.scheduler.{find_by_text, remove_by_text}
  - Voice: "отмени напоминание про воду" / "удали задачу про разминку"
  - 3 new unit tests (52 total in jarvis-core).

TTS pre-warm (crates/jarvis-app/src/main.rs)
  - Call jarvis_core::tts::backend() once on startup so the first speak()
    doesn't pay Piper binary discovery + voice loading cost. Cuts first-speak
    latency by ~150-300ms depending on backend.

Build: cargo build --release -p jarvis-app -p jarvis-gui both green.
Tests: 52/52 jarvis-core unit tests pass.

resources/commands/media_keys/command.toml

@@ -0,0 +1,80 @@
+# Media playback control via Windows virtual media keys.
+# Works with anything that listens to global media keys: Spotify, Yandex Music,
+# YouTube in browser (when tab is focused), Foobar2000, Winamp, etc.
+# No OAuth, no API keys — just sends VK_MEDIA_* via PowerShell SendKeys.
+
+[[commands]]
+id = "media.play_pause"
+type = "lua"
+script = "key.lua"
+sandbox = "full"
+timeout = 2000
+
+[commands.phrases]
+ru = [
+    "пауза",
+    "продолжи",
+    "плей",
+    "включи музыку",
+    "выключи музыку",
+    "приостанови музыку",
+    "снова музыку",
+]
+en = ["pause", "play", "resume music", "stop music"]
+ua = ["пауза", "продовжи"]
+
+
+[[commands]]
+id = "media.next"
+type = "lua"
+script = "key.lua"
+sandbox = "full"
+timeout = 2000
+
+[commands.phrases]
+ru = [
+    "следующий трек",
+    "следующую песню",
+    "перемотай вперёд",
+    "вперёд трек",
+    "переключи на следующую",
+    "next",
+]
+en = ["next track", "next song", "skip"]
+ua = ["наступний трек"]
+
+
+[[commands]]
+id = "media.prev"
+type = "lua"
+script = "key.lua"
+sandbox = "full"
+timeout = 2000
+
+[commands.phrases]
+ru = [
+    "предыдущий трек",
+    "предыдущую песню",
+    "перемотай назад",
+    "назад трек",
+    "переключи на предыдущую",
+    "previous",
+]
+en = ["previous track", "previous song", "back"]
+ua = ["попередній трек"]
+
+
+[[commands]]
+id = "media.stop"
+type = "lua"
+script = "key.lua"
+sandbox = "full"
+timeout = 2000
+
+[commands.phrases]
+ru = [
+    "стоп музыка",
+    "останови плеер",
+]
+en = ["stop", "stop player"]
+ua = ["зупини плеєр"]