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/device_service.py

@@ -0,0 +1,152 @@
+"""High-level wafer device communication service.
+
+Coordinates port enumeration, detect, read, and erase operations
+using settings from LocalSettingsModel.  Designed to be called from
+QML via @Slot methods on a QObject controller.
+"""
+
+import logging
+from pathlib import Path
+from typing import Optional
+
+import serial.tools.list_ports
+
+from pygui.backend.local_settings import LocalSettings
+from pygui.serialcomm.serial_port import SerialPort, WaferInfo
+
+log = logging.getLogger(__name__)
+
+
+class DeviceService:
+    """High-level wafer device communication."""
+
+    # Expected hex string lengths for known wafer families
+    # P wafer: 393216 hex chars (196608 bytes)
+    # X wafer: 1310720 hex chars (655360 bytes)
+    EXPECTED_HEX_LENGTHS = {
+        "P": 393216,
+        "X": 1310720,
+    }
+
+    def __init__(self, settings: LocalSettings) -> None:
+        self._settings = settings
+
+    # ------------------------------------------------------------------
+    # Public API
+    # ------------------------------------------------------------------
+
+    def enumerate_ports(self) -> list[str]:
+        """Return list of available serial port names."""
+        return [p.device for p in serial.tools.list_ports.comports()]
+
+    def detect_wafer(self, port: str) -> Optional[WaferInfo]:
+        """Try to detect a wafer on the given port.
+
+        Opens the port, sends s0 command, parses response.
+        Returns WaferInfo if detected, None otherwise.
+        """
+        try:
+            sp = SerialPort(port)
+            info = sp.detect(self._settings.wafer_detect_timeout)
+            return info
+        except Exception as exc:
+            log.error("Detect failed on %s: %s", port, exc)
+            return None
+
+    SCAN_PER_PORT_TIMEOUT_MS = 1500
+    """Per-port timeout while scanning. Wafer responds fast or not at all."""
+
+    def detect_all_ports(self) -> tuple[Optional[str], Optional[WaferInfo]]:
+        """Scan every available serial port until one responds to s0.
+
+        Mirrors C# chkConnection(): enumerates all ports, tries each one,
+        returns the first that responds with a valid 1024-byte payload.
+
+        Uses a short per-port timeout (SCAN_PER_PORT_TIMEOUT_MS) so a
+        machine with many serial devices (Bluetooth, debug consoles, etc.)
+        doesn't take 30+ seconds to scan. The configured wafer_detect_timeout
+        is reserved for confirmed-port operations.
+
+        Returns:
+            (port_name, WaferInfo) on success, (None, None) if nothing found.
+        """
+        ports = self.enumerate_ports()
+        log.info("Scanning %d port(s) for wafer ...", len(ports))
+        for port in ports:
+            log.info("  trying %s ...", port)
+            try:
+                sp = SerialPort(port)
+                info = sp.detect(self.SCAN_PER_PORT_TIMEOUT_MS)
+            except Exception as exc:
+                log.warning("  %s: %s", port, exc)
+                info = None
+            if info is not None:
+                log.info("  found wafer on %s (family=%s)", port, info.family_code)
+                return port, info
+        log.info("  no wafer detected on any port")
+        return None, None
+
+    def read_wafer_data(
+        self,
+        port: str,
+        family_code: str = "",
+        cmd: str = "D1",
+        timeout_ms: int | None = None,
+        retries: int | None = None,
+    ) -> Optional[bytes]:
+        """Full read workflow: open → send command → retry → close.
+
+        Args:
+            port: Serial port name (e.g. "/dev/ttyUSB0").
+            family_code: Wafer family ("P", "X", etc.) for size validation.
+            cmd: Command string ("D1" for sensor data, "F1" for debug data).
+            timeout_ms: Read timeout in ms (uses settings default if None).
+            retries: Max retries on bad read (uses settings default if None).
+
+        Returns:
+            Raw bytes from the wafer, or None on failure.
+        """
+        if timeout_ms is None:
+            timeout_ms = self._settings.wafer_read_timeout
+        if retries is None:
+            retries = self._settings.wafer_read_retries
+
+        # Determine expected hex string length.
+        # get_wafer_data_size returns BYTES; hex string is 2× that.
+        if family_code:
+            expected_hex_len = self._settings.get_wafer_data_size(family_code) * 2
+        else:
+            expected_hex_len = 0  # no validation
+
+        try:
+            sp = SerialPort(port)
+            data = sp.read_memory(
+                cmd=cmd,
+                expected_hex_len=expected_hex_len,
+                timeout_ms=timeout_ms,
+                retries=retries,
+            )
+            if data is not None:
+                log.info("Read %d bytes from wafer on %s", len(data), port)
+            else:
+                log.warning("Read returned no data from %s", port)
+            return data
+        except Exception as exc:
+            log.error("Read failed on %s: %s", port, exc)
+            return None
+
+    def erase_wafer(self, port: str) -> bool:
+        """Send p1 erase command. Wafer takes ~15s to erase.
+
+        Returns True if command was sent successfully.
+        """
+        try:
+            sp = SerialPort(port)
+            return sp.erase_memory()
+        except Exception as exc:
+            log.error("Erase failed on %s: %s", port, exc)
+            return False
+
+    def get_expected_hex_length(self, family_code: str) -> int:
+        """Return expected hex string length for a wafer family."""
+        return self._settings.get_wafer_data_size(family_code)