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>

sc3_aui_app.py

@@ -917,6 +917,7 @@ class MainWindow(QMainWindow):
         self.row_spacing_edit.setText("0.250")
         self.scan_prefix_edit.setText("scan")
         self.scan_save_dir_edit.setText(DEFAULTS.save_dir)
+        self.burst_mode_check.setChecked(DEFAULTS.burst_mode)
 
         # Hide the old T3R manual controls; the connect toggle becomes the panel button.
         for w in (
@@ -977,6 +978,7 @@ class MainWindow(QMainWindow):
         self.t3r_comport_edit.editingFinished.connect(self._persist_defaults)
         self.bbd202_comport_edit.editingFinished.connect(self._persist_defaults)
         self.oscope_ip_edit.editingFinished.connect(self._persist_defaults)
+        self.burst_mode_check.toggled.connect(self._persist_defaults)
 
         # Camera
         self.show_camera_toggle.toggled.connect(self._on_camera_toggle)
@@ -1134,6 +1136,7 @@ class MainWindow(QMainWindow):
         DEFAULTS.bbd_port = self.bbd202_comport_edit.text().strip()
         DEFAULTS.oscope_ip = self.oscope_ip_edit.text().strip()
         DEFAULTS.save_dir = self.scan_save_dir_edit.text().strip()
+        DEFAULTS.burst_mode = self.burst_mode_check.isChecked()
         DEFAULTS.save()
 
     # ── Camera toggle ─────────────────────────────────────────────────────────
@@ -1257,6 +1260,7 @@ class MainWindow(QMainWindow):
             # a worker concern, not the engine's.
             on_scan_active=lambda active: setattr(
                 self._bbd_worker, "scanning_active", active),
+            burst_mode=self.burst_mode_check.isChecked(),
         )
         self._scan_worker.moveToThread(self._scan_thread)
         self._scan_thread.started.connect(self._scan_worker.run)