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>

tests/test_config.py

@@ -0,0 +1,50 @@
+"""core.config: round-trip, tolerance, and the helios_port regression.
+
+The old dict-based writer rebuilt the JSON from only the main window's
+fields, silently discarding helios_port every time a port was edited.
+ScanDefaults.save() always writes every field.
+"""
+import json
+
+from core.config import ScanDefaults
+
+
+def test_roundtrip(tmp_path):
+    p = tmp_path / "defaults.json"
+    d = ScanDefaults(t3r_port="/dev/ttyACM3", helios_port="/dev/ttyUSB9")
+    d.save(p)
+    loaded = ScanDefaults.load(p)
+    assert loaded == d
+
+
+def test_missing_file_creates_defaults(tmp_path):
+    p = tmp_path / "defaults.json"
+    d = ScanDefaults.load(p)
+    assert d == ScanDefaults()
+    assert p.exists()
+
+
+def test_corrupt_file_falls_back(tmp_path):
+    p = tmp_path / "defaults.json"
+    p.write_text("{not json")
+    assert ScanDefaults.load(p) == ScanDefaults()
+
+
+def test_unknown_keys_ignored(tmp_path):
+    p = tmp_path / "defaults.json"
+    p.write_text(json.dumps({"t3r_port": "/dev/ttyACM7", "laser_freq_hz": 20000.0}))
+    d = ScanDefaults.load(p)
+    assert d.t3r_port == "/dev/ttyACM7"
+    assert d.helios_port == ScanDefaults().helios_port
+
+
+def test_helios_port_survives_partial_update(tmp_path):
+    """Regression: editing main-window ports must not clobber helios_port."""
+    p = tmp_path / "defaults.json"
+    ScanDefaults(helios_port="/dev/ttyUSB7").save(p)
+
+    d = ScanDefaults.load(p)
+    d.t3r_port = "/dev/ttyACM1"     # what _persist_defaults does
+    d.save(p)
+
+    assert ScanDefaults.load(p).helios_port == "/dev/ttyUSB7"