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>

core/config.py

@@ -0,0 +1,47 @@
+"""Persisted user defaults (ports, scope IP, save directory).
+
+One flat JSON file, one dataclass.  ``save()`` always writes every field, so
+a partial UI update can never silently drop another field's saved value
+(which is exactly what the old dict-based writer did to helios_port).
+"""
+from __future__ import annotations
+
+import json
+import logging
+from dataclasses import asdict, dataclass, fields
+from pathlib import Path
+
+logger = logging.getLogger(__name__)
+
+DEFAULTS_PATH = Path(__file__).resolve().parent.parent / "aui_defaults.json"
+
+
+@dataclass
+class ScanDefaults:
+    t3r_port: str = "/dev/ttyUSB0"
+    bbd_port: str = "/dev/ttyUSB1"
+    oscope_ip: str = "192.168.0.1"
+    save_dir: str = str(DEFAULTS_PATH.parent / "scans")
+    helios_port: str = "/dev/ttyUSB2"
+
+    @classmethod
+    def load(cls, path: Path = DEFAULTS_PATH) -> "ScanDefaults":
+        """Load defaults, tolerating a missing/corrupt file and unknown keys."""
+        if path.exists():
+            try:
+                with open(path) as f:
+                    data = json.load(f)
+                known = {f.name for f in fields(cls)}
+                return cls(**{k: v for k, v in data.items() if k in known})
+            except (OSError, ValueError, TypeError) as e:
+                logger.warning("Could not load %s (%s); using fallback defaults", path, e)
+        inst = cls()
+        inst.save(path)
+        return inst
+
+    def save(self, path: Path = DEFAULTS_PATH) -> None:
+        try:
+            with open(path, "w") as f:
+                json.dump(asdict(self), f, indent=2)
+        except OSError as e:
+            logger.warning("Could not save defaults to %s: %s", path, e)