Add GUI control panel with live measurements and full instrument control

New `mppt-gui` command launches a tkinter app with:
- Live numeric readouts for all 13 measurement channels
- 4-panel real-time matplotlib graphs (power, efficiency, voltage, current)
- Full supply control: voltage, current, OVP, slew, output on/off
- Full load control: mode (CC/CR/CV/CP), setpoint, slew, load on/off
- Meter settings: wiring mode, response speed, coupling per channel
- CSV data logging with start/stop control
- Red SAFE OFF button (also Escape key) bypasses command queue
- Threaded architecture: instrument I/O never blocks the GUI
- Fix apply() silently failing: use set_current + set_voltage instead

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -25,3 +25,4 @@ packages = ["testbench"]
 
 [project.scripts]
 mppt = "testbench.cli:main"
+mppt-gui = "testbench.gui:main"