fix(example): J-key voice command ack protocol to stop stale repeats

[cavonlee]

Problem

Client reported that sc.get("J") keeps returning the last voice recognition result forever (see PiCar-X / PiDog app control demos). Root cause on the demo side: the speak variable is assigned once and never reset, so the car keeps executing the last voice command.

Fix

Rework the J-key voice handling in examples/picarx_control.py:

1. Edge-triggered execution — only execute when J transitions from empty/None to a command, so repeated identical commands (e.g. consecutive "forward") trigger again.
2. Ack protocol with the App — the App keeps the voice command in its send buffer until the device consumes it:
   • Device receives command → returns J: 1 (ack, action in progress)
   • Device executes the action (~0.3s, adjustable)
   • Device returns J: 0 (action done) → App clears the pending command

This stops the stale repeat and makes repeated voice commands work.

Testing

1. Run picarx_control.py on the Pi, connect the SunFounder Controller App
2. Speak a command (e.g. "forward") — car should move briefly then stop
3. Speak the same command again — car should move again (repeat works)
4. Check App console for STT read changed (value 1 and 0) + STT value clear

Version bumped to 0.0.4 (0.0.3 is claimed by the open debug-logging PR #3).