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/CHEATSHEET.md

@@ -0,0 +1,12 @@
+# Cheatsheet
+
+```
+make setup           setup venv + deps
+make run             launch tuimble
+make test            run tests
+make lint            check code style
+
+q                    quit
+space                push-to-talk (hold)
+Enter                send message
+```