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/contour_models.py

@@ -0,0 +1,26 @@
+from dataclasses import dataclass, field
+from typing import List
+
+
+# ===== Single Contour Segment =====
+@dataclass
+class ContourSegment:
+    start_x: float
+    start_y: float
+    end_x: float
+    end_y: float
+
+    @property
+    def start(self):
+        return (self.start_x, self.start_y)
+
+    @property
+    def end(self):
+        return (self.end_x, self.end_y)
+
+
+# ===== Contour Line =====
+@dataclass
+class ContourLine:
+    level: float
+    segments: List[ContourSegment] = field(default_factory=list)