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/data_segment.py

@@ -0,0 +1,55 @@
+from datetime import datetime, timedelta
+
+
+# ===== Time-Indexed Data Segment =====
+class DataSegment:
+    # ===== Lifecycle =====
+    def __init__(
+        self,
+        full_data: list[float],
+        start_time: datetime,
+        start_index: int,
+        end_index: int,
+    ):
+        if not isinstance(start_index, int):
+            raise TypeError("start_index must be an integer")
+        if not isinstance(end_index, int):
+            raise TypeError("end_index must be an integer")
+        """
+        full_data: The complete list of sensor readings.
+        start_time: The timestamp of the very first reading in full_data.
+        start_index: The offset (in seconds) from start_time to the beginning of this segment.
+        end_index: The offset (in seconds) from start_time to the end of this segment.
+        """
+        self.full_data = full_data
+        self._base_start_time = start_time
+        self._start_index = start_index
+        self._end_index = end_index
+        self.chamber = ""
+        self.notes = ""
+
+    # ===== Index Properties =====
+    @property
+    def start_index(self) -> int:
+        return self._start_index
+
+    @start_index.setter
+    def start_index(self, value: int):
+        self._start_index = value
+
+    @property
+    def end_index(self) -> int:
+        return self._end_index
+
+    @end_index.setter
+    def end_index(self, value: int):
+        self._end_index = value
+
+    # ===== Derived Time Properties =====
+    @property
+    def start_time(self) -> datetime:
+        return self._base_start_time + timedelta(seconds=self.start_index)
+
+    @property
+    def end_time(self) -> datetime:
+        return self._base_start_time + timedelta(seconds=self.end_index)