docs: add pitch shifting documentation

F6/F7 keybindings, audio.pitch config option, voice pitch section
in USAGE.md.  Updated profiling notes to reflect per-thread coverage.

README.md

@@ -11,6 +11,7 @@ TUI Mumble client with voice support and push-to-talk.
 - Chat input history (Up/Down arrow navigation)
 - Self-deafen toggle
 - Volume control (input/output gain)
+- Voice pitch shifting (deeper/higher)
 - Client certificate authentication
 - Auto-reconnect on network loss
 - Config hot-reload (F5)
@@ -25,6 +26,8 @@ TUI Mumble client with voice support and push-to-talk.
 | `F3` | Cycle input volume |
 | `F4` | Push-to-talk (configurable) |
 | `F5` | Reload config from disk |
+| `F6` | Pitch down (1 semitone) |
+| `F7` | Pitch up (1 semitone) |
 | `Enter` | Send message / join channel (sidebar) |
 | `Up` | Previous message (input) / previous channel (sidebar) |
 | `Down` | Next message (input) / next channel (sidebar) |
@@ -57,6 +60,7 @@ username = "myname"
 [audio]
 # output_gain = 1.0
 # input_gain = 1.0
+# pitch = 0          # semitones, -12 to +12
 
 [ptt]
 key = "f4"