Restructure into src/ layout under pygui package

Move all application source under src/pygui/ and rewire imports,
build config, and QML module path to match.
- Relocate backend/, serialcomm/, and the ISC QML module into
  src/pygui/; convert main.py into pygui/__main__.py with a main()
  entry point (run via `python -m pygui` or the new `isc` script)
- Rewrite absolute imports: backend.* -> pygui.backend.*,
  serialcomm.* -> pygui.serialcomm.* (source + tests)
- Move app icons (isc.ico/icns) into packaging/
- Update README and ISC.qmlproject to the new paths

src/pygui/serialcomm/__init__.py

@@ -0,0 +1,6 @@
+"""Serial port communication layer for the temperature-sensing wafer."""
+
+from pygui.serialcomm.device_service import DeviceService
+from pygui.serialcomm.serial_port import SerialPort, WaferInfo
+
+__all__ = ["DeviceService", "SerialPort", "WaferInfo"]