Add UART diagnostic capture tool for board-day bring-up

- Python/pyserial script captures STM32 USART3 DIAG output (115200 8N1)
- Auto-detects serial port on macOS (ST-Link VCP, FTDI, CH340, CP210x)
- Color-coded terminal output by subsystem and severity
- Simultaneous logging to timestamped file in logs/
- Filtering by subsystem tag (--filter LO,PA) or severity (--errors-only)
- Parses all DIAG macro formats: DIAG, DIAG_WARN, DIAG_ERR, DIAG_SECTION
- Capture stats summary on exit (line counts by subsystem, error/warning totals)
- logs/ added to .gitignore

.gitignore

@@ -54,3 +54,6 @@ __pycache__/
 
 # Local timing closure report snapshots
 build*_reports/
+
+# UART capture logs (generated by tools/uart_capture.py)
+logs/