Make row packing toggleable: pad (default) or strict abort

A mis-triggered row cannot be written as it arrived — v6 declares n_frames per
row in the header and has no per-row length field, so a short or long row
would shift every later row in the file. Until now the only policy was to
square it up, which keeps the scan running but leaves the affected row
indistinguishable from a good one afterwards: nothing in the file records that
it was padded.

strict_rows selects the other trade. On any frame-count mismatch the scan
stops instead of writing the row, so a data run either produces rows that mean
what the header says they mean or fails loudly. Default stays pad, so existing
behaviour is unchanged.

_warn_frame_delta becomes _check_frame_delta, since it now decides rather than
just reports. Both acquisition paths already call it before writing anything
for the row (CH1 leads SCAN_CHANNELS, and the burst path checks every row up
front), so an abort leaves the file on a whole-row boundary rather than a
half-written row — test_strict_row_packing_writes_nothing_for_the_failed_row
pins that.

Plumbed through QtScanController to a checkbox in the scan panel, persisted in
ScanDefaults alongside burst_mode. scan_format.md documents both policies and
notes that the choice is not recorded in the file.

The row-clipping setup in the padding test is now a _clip_one_row helper,
reused by the strict tests. 92 tests passing, ruff clean.

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

sc3-aui-main.ui

@@ -1007,6 +1007,16 @@
         </property>
        </widget>
       </item>
+      <item>
+       <widget class="QCheckBox" name="strict_rows_check">
+        <property name="toolTip">
+         <string>Stop the scan if a row does not acquire the expected number of frames, instead of zero-padding a short row or truncating a long one. Use for data runs where a silently squared-up row would be worse than a failed scan.</string>
+        </property>
+        <property name="text">
+         <string>Strict row packing (abort on frame-count mismatch)</string>
+        </property>
+       </widget>
+      </item>
       <item>
        <widget class="QPushButton" name="start_scan_btn">
         <property name="text">