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-main.ui

@@ -997,6 +997,16 @@
         </property>
        </widget>
       </item>
+      <item>
+       <widget class="QCheckBox" name="burst_mode_check">
+        <property name="toolTip">
+         <string>Acquire as many whole rows per FastFrame acquisition as the scope can hold, and transfer each burst in one CURVe? transaction. The stage trigger output is gated off for the flyback between rows.</string>
+        </property>
+        <property name="text">
+         <string>Burst acquisition (multi-row FastFrame)</string>
+        </property>
+       </widget>
+      </item>
       <item>
        <widget class="QPushButton" name="start_scan_btn">
         <property name="text">