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/backend/frame.py

@@ -0,0 +1,11 @@
+from __future__ import annotations
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class Frame:
+    """One sample across all sensors ata a point in time"""
+
+    seq: int
+    t: float
+    values: list[float]