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/serial_util.py

@@ -0,0 +1,44 @@
+"""Shared serial-port helpers: 8N1 open and scored port enumeration.
+
+Qt-free — GUI code adapts the (device, label) list into its own widgets.
+"""
+from __future__ import annotations
+
+import serial
+from serial.tools import list_ports
+
+# Substrings that suggest a USB-serial adapter we actually talk to
+# (ESP32-based T3R, CP210x/CH340 dongles, CDC-ACM devices); matching ports
+# sort first in pickers.
+DEVICE_HINTS = ("esp32", "jtag", "espressif", "usb serial", "cp210", "ch340", "cdc")
+
+
+def open_8n1(port: str, baudrate: int, timeout: float,
+             write_timeout: float | None = None) -> serial.Serial:
+    """Open a serial port with the 8N1 framing every device here uses."""
+    return serial.Serial(
+        port=port,
+        baudrate=baudrate,
+        bytesize=serial.EIGHTBITS,
+        parity=serial.PARITY_NONE,
+        stopbits=serial.STOPBITS_ONE,
+        timeout=timeout,
+        write_timeout=write_timeout,
+    )
+
+
+def scored_ports() -> list[tuple[str, str]]:
+    """Enumerate serial ports as (device, human label), likeliest-first."""
+    ports = list(list_ports.comports())
+
+    def score(p):
+        text = f"{p.description} {p.manufacturer or ''} {p.product or ''}".lower()
+        return -sum(h in text for h in DEVICE_HINTS)
+
+    ports.sort(key=score)
+    return [(p.device, f"{p.device} — {p.description or p.device}") for p in ports]
+
+
+def list_port_devices() -> list[str]:
+    """Plain device-path list, likeliest-first."""
+    return [dev for dev, _ in scored_ports()]