Build threaded bridge architecture for control surface mode

[joaodotwork]

Description

Implement the three-thread architecture: mido MIDI callbacks, OSC server daemon thread, and main thread for rendering + USB display transfer.

Tasks

• Main thread: Push2 init, render loop at 30fps, display_frame() calls
• mido thread: encoder/button callbacks (managed by push2-python/RtMidi)
• OSC server thread: ThreadingOSCUDPServer as daemon thread
• Shared ParameterState object with proper locking
• Dirty-flag rendering — only redraw when parameters change
• Keep-alive rendering at 10fps minimum (prevent 2-second display timeout)
• Clean shutdown across all threads on SIGINT
• Mode switching between video bridge mode and control surface mode

Acceptance Criteria

• All three threads run concurrently without deadlocks
• Display updates at 30fps when parameters are changing
• Display doesn't black out during idle periods (keep-alive)
• Clean shutdown releases USB, stops OSC server, closes MIDI

Technical Notes

Main Thread: Push2 init → render loop (30fps) → display_frame()
mido Thread: @on_encoder_rotated → state.set_from_encoder() → osc_client.send()
OSC Thread:  ThreadingOSCUDPServer → state.set_from_osc()

• display_frame() is blocking USB (~2-5ms)
• Push2() must be created on main thread