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>

scan_format.md

@@ -183,18 +183,48 @@ using that angle's `x_start` from the Per-Angle Geometry Table (not
 
 ---
 
-## Acquisition Settings (fixed by sc3_aui_app.py)
+## Acquisition Settings (fixed by core/scope_sras.py)
 
-| Parameter            | Value                        |
-|-----------------------|------------------------------|
-| Oscilloscope trigger | CH2, rising edge, 1.24 V     |
-| Trigger offset       | 0 % (trigger at left edge)   |
-| Sample rate          | 6.25 GS/s  (160 ps/sample)   |
-| Channels recorded    | CH1, CH3, CH4                |
-| Stage X velocity     | 100 mm/s                     |
-| Stage X acceleration | 1500 mm/s²                   |
-| Stage X trigger out  | Logic-high at max velocity   |
-| Acquisition mode     | FastFrame, Normal trigger    |
+| Parameter            | Value                                    |
+|-----------------------|------------------------------------------|
+| Setup trigger        | CH2, rising edge, 0.500 V (`TRIG_LEVEL_V`) |
+| Scan trigger         | Logic AND, CH2 HIGH ∧ CH3 HIGH, 0.500 V  |
+| Horizontal position  | 30 (`HORizontal:POSition`)               |
+| Sample rate          | 6.25 GS/s  (160 ps/sample)               |
+| Transfer format      | `DATa:ENCdg RIBinary`, `DATa:WIDth 1`    |
+| Channels recorded    | CH1, CH3, CH4                            |
+| Stage X velocity     | 100 mm/s                                 |
+| Stage X acceleration | 1500 mm/s²                               |
+| Stage X trigger out  | Logic-high at max velocity (`TRIGOUT_MAXV`) |
+| Acquisition mode     | FastFrame, Normal trigger                |
+
+None of these are stored in the file, so they do not affect byte layout — but
+they do set where the acoustic packet lands inside each frame.  Read them from
+`core/scope_sras.py`; earlier revisions of this table drifted from the code.
+
+---
+
+## Acquisition Paths
+
+Two acquisition strategies write **byte-identical** files; the choice is a
+runtime flag (`ScanEngine(burst_mode=…)`, exposed as a checkbox in the app) and
+is not recorded in the file.
+
+| | Per-row (default) | Burst |
+|---|---|---|
+| FastFrame acquisitions | one per row | one per `floor(max_frames / n_frames)` rows |
+| Curve transfers | one per channel per row | one per channel per burst |
+| Stage X trigger out | armed for the whole scan | armed per acquiring pass, dropped for the flyback |
+
+Burst mode runs a single acquisition across several rows, so the return move
+must not trigger: the trigger output is dropped before each flyback and
+re-armed for each acquiring pass.  Row boundaries inside the burst come from
+`ACQuire:NUMFRAMESACQuired?` sampled after each pass — the burst itself carries
+no row markers.  See `core/scope_burst.py`.
+
+Either path squares each row up to the declared `n_frames` (zero-padding a
+short row, dropping the tail of a long one), because the format has no per-row
+length field and a mismatch would shift every later row.
 
 ---