Expose burst acquisition in the app; refresh scan_format acquisition docs

Wires the burst_mode flag through QtScanController to ScanEngine and adds a
checkbox to the scan panel. The setting persists via ScanDefaults like the
other scan fields, defaulting to off — per-row acquisition stays the default
path until burst mode has run on the rig and the gate-off preflight has
settled which TRIGOUT value idles the pin low.

scan_format.md — the acquisition settings table had drifted from the code it
claimed to describe: it attributed the settings to sc3_aui_app.py (they moved
to core/scope_sras.py in the Phase 2 extraction), listed a 1.24 V trigger
level and 0 % offset where the code sets 0.500 V and HORizontal:POSition 30,
and did not mention the logic-AND scan trigger at all. Corrected, pointed at
the module that actually owns them, and noted that none of it affects byte
layout — only where the acoustic packet lands inside a frame.

Added an acquisition-paths section: the two paths write byte-identical files
and the choice is a runtime flag that is not recorded in the file, so a
reader never needs to care which produced it. Documents where row boundaries
come from in a burst and that either path squares rows up to n_frames.

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

core/config.py

@@ -23,6 +23,7 @@ class ScanDefaults:
     oscope_ip: str = "192.168.0.1"
     save_dir: str = str(DEFAULTS_PATH.parent / "scans")
     helios_port: str = "/dev/ttyUSB2"
+    burst_mode: bool = False
 
     @classmethod
     def load(cls, path: Path = DEFAULTS_PATH) -> "ScanDefaults":