Phase 2: extract headless core modules (sras_format, scan_geometry, config)

- core/sras_format.py: THE v6 implementation — create_scan_file (writer,
  byte-identical to the old one, enforced against the Phase-0 goldens),
  SrasFile parser with frontier/truncation walk, and zero-copy mmap
  load_angle/load_row views for multi-GB files
- core/scan_geometry.py: ScanPlan/AngleGeometry dataclasses, build_plan
  (rotated-bbox trig from MainWindow._build_scan_params), travel-limit
  validate_plan (limits now a StageLimits dataclass, not literals buried
  in the worker), format_eta + EtaEstimator (bounded deque)
- core/config.py: ScanDefaults dataclass replaces the module-import-time
  dict globals. FIXES: editing any main-window port used to rewrite
  aui_defaults.json without helios_port, silently reverting the Helios
  port every time (test_helios_port_survives_partial_update covers it).
  Also drops the inert laser_freq_hz plumbing — scans always used the
  LASER_FREQ_HZ constant.
- hardware/serial_util.py: shared 8N1 open + scored port enumeration
  (promoted from t3r_control_panel); helios_laser and the panel use it
- sc3_aui_app.py and sras_scan_manager.py migrated onto core (three
  format implementations down to one); ScanWorker now takes a ScanPlan
- tests: byte-identical writer vs golden, frontier over every truncation
  variant, mmap==eager, geometry vs golden fixtures + invariants, config
  round-trip. 28 passing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

hardware/helios_laser.py

@@ -3,12 +3,13 @@ Helios Laser System Driver
 Basic implementation for controlling the Helios pulsed laser.
 """
 
-import serial
 import time
 import logging
 from typing import Optional, List
 from enum import Enum
 
+from hardware.serial_util import open_8n1
+
 logger = logging.getLogger(__name__)
 
 
@@ -42,10 +43,9 @@ class HeliosLaser:
 
     @staticmethod
     def list_available_ports() -> List[str]:
-        """List available serial ports"""
-        import serial.tools.list_ports
-        ports = serial.tools.list_ports.comports()
-        return [port.device for port in ports]
+        """List available serial ports, likeliest devices first."""
+        from hardware.serial_util import list_port_devices
+        return list_port_devices()
 
     def connect(self, port: str = None) -> bool:
         """
@@ -65,14 +65,7 @@ class HeliosLaser:
             return False
 
         try:
-            self.serial = serial.Serial(
-                port=self.port,
-                baudrate=9600,
-                bytesize=serial.EIGHTBITS,
-                parity=serial.PARITY_NONE,
-                stopbits=serial.STOPBITS_ONE,
-                timeout=self.timeout
-            )
+            self.serial = open_8n1(self.port, baudrate=9600, timeout=self.timeout)
             time.sleep(0.1)  # Allow time for connection to stabilize
             self.is_connected = True
             logger.info(f"Connected to Helios laser on {self.port}")