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.

crates/jarvis-core/src/llm/mod.rs

@@ -2,7 +2,65 @@ mod client;
 mod history;
 
 pub use client::{
-    ChatMessage, ConfigError, LlmClient, LlmError,
+    ChatMessage, ConfigError, LlmBackend, LlmClient, LlmError,
     DEFAULT_BASE_URL, DEFAULT_MODEL, ENV_BASE_URL, ENV_MODEL, ENV_TOKEN,
+    OLLAMA_DEFAULT_BASE_URL, OLLAMA_DEFAULT_MODEL,
 };
 pub use history::ConversationHistory;
+
+use once_cell::sync::Lazy;
+use parking_lot::RwLock;
+use std::sync::Arc;
+
+/// Shared mutable LLM client. Modules read via `current()`; the user can
+/// hot-swap backends at runtime via `swap_to(LlmBackend::Ollama)` etc.
+///
+/// `None` means LLM is disabled (no GROQ_TOKEN and no Ollama reachable, or
+/// `init_global()` was never called).
+static GLOBAL: Lazy<RwLock<Option<Arc<LlmClient>>>> = Lazy::new(|| RwLock::new(None));
+
+/// Initialise the global client from env vars (`JARVIS_LLM`, `GROQ_TOKEN`, ...).
+/// Idempotent — replaces any previous global. Returns Ok even when the chosen
+/// backend can't be probed: an Ollama server may not be running yet but we
+/// stash the client anyway and let calls fail on first request.
+pub fn init_global() -> Result<(), ConfigError> {
+    let c = LlmClient::from_env()?;
+    log::info!("LLM global initialised: backend={}, model={}", c.backend().name(), c.model());
+    *GLOBAL.write() = Some(Arc::new(c));
+    Ok(())
+}
+
+/// Return a clone of the active client. Cheap (Arc).
+pub fn current() -> Option<Arc<LlmClient>> {
+    GLOBAL.read().clone()
+}
+
+/// Name of the currently active backend, or "none" if not initialised.
+pub fn current_backend_name() -> &'static str {
+    match GLOBAL.read().as_ref().map(|c| c.backend()) {
+        Some(LlmBackend::Groq) => "groq",
+        Some(LlmBackend::Ollama) => "ollama",
+        None => "none",
+    }
+}
+
+/// Hot-swap to a different backend. Returns the new backend's name on success.
+pub fn swap_to(backend: LlmBackend) -> Result<&'static str, ConfigError> {
+    let c = match backend {
+        LlmBackend::Groq => LlmClient::groq()?,
+        LlmBackend::Ollama => LlmClient::ollama(),
+    };
+    let name = c.backend().name();
+    log::info!("LLM swapped → backend={}, model={}", name, c.model());
+    *GLOBAL.write() = Some(Arc::new(c));
+    Ok(name)
+}
+
+/// Parse a backend name (case-insensitive) into the enum.
+pub fn parse_backend(name: &str) -> Option<LlmBackend> {
+    match name.trim().to_lowercase().as_str() {
+        "groq" | "cloud" | "облако" | "клауд" => Some(LlmBackend::Groq),
+        "ollama" | "local" | "локал" | "локальн" | "локальный" => Some(LlmBackend::Ollama),
+        _ => None,
+    }
+}