feat: scaffold tuimble TUI mumble client

Core modules: TUI app (textual), mumble protocol client,
audio pipeline (sounddevice + opus), push-to-talk with
kitty protocol / evdev / toggle backends. Config via TOML.

docs/USAGE.md

@@ -0,0 +1,27 @@
+# Usage
+
+## Running
+
+```sh
+tuimble                          # uses ~/.config/tuimble/config.toml
+tuimble --host mumble.example.com --user myname
+```
+
+## Key Bindings
+
+| Key | Action |
+|-----|--------|
+| `q` | Quit |
+| `space` | Push-to-talk (configurable) |
+| `Enter` | Send message |
+| `Ctrl+C` | Quit |
+
+## Push-to-Talk Modes
+
+- **hold** — hold key to transmit, release to stop (default)
+- **toggle** — press to start, press again to stop
+
+## Configuration
+
+See `~/.config/tuimble/config.toml`. All fields are optional;
+defaults connect to localhost:64738.