add --cprofile flag with periodic dump

docs/USAGE.md

@@ -22,6 +22,26 @@ tuimble --host mumble.example.com --user myname
 - **toggle** — press to start, press again to stop (default)
 - **hold** — hold key to transmit, release to stop (requires evdev)
 
+## Profiling
+
+```sh
+tuimble --cprofile                    # saves to ~/.config/tuimble/profile.prof
+tuimble --cprofile /tmp/tuimble.prof  # saves to custom path
+```
+
+Profile data is dumped every 30 seconds and on exit, so snapshots
+are available even after a crash or `kill`. Output is standard `.prof`
+format:
+
+```sh
+python3 -m pstats /tmp/tuimble.prof   # interactive explorer
+# or install snakeviz for a browser-based flamegraph:
+# pip install snakeviz && snakeviz /tmp/tuimble.prof
+```
+
+Note: cProfile captures the main thread only. Background workers
+started with `@work(thread=True)` are not included.
+
 ## Configuration
 
 See `~/.config/tuimble/config.toml`. All fields are optional;