Per-angle background capture: v7/v11 .sras layout

A multi-angle scan runs for hours, but every angle was referenced against
one background captured before the first row of the first angle. That
reference has drifted by the last angle, and comparing angles — the whole
point of a multi-angle scan — was comparing each one against a noise floor
measured at whichever angle came first.

Every angle now captures its own. Before each angle's rows, the operator is
prompted to switch the Genesis laser off, the engine averages a fresh CH1
record, and the operator switches it back on. The data block therefore reads
[background][scan][background][scan] …, one pair per angle.

Format v7 (scan) and v11 (SAW check) carry the background inside the data
block, one length-prefixed block ahead of each angle's rows; the single
block that sat between the preambles and the data is gone. Per-angle offsets
now come from a walk of the data block at parse time rather than arithmetic
over the geometry table, and an angle whose background is not fully on disk
is the frontier — nothing of it was written yet.

v6/v10 files still read: SrasFile hands their one background to every angle,
so readers never branch on the version. Nothing writes them, and a resume
refuses them, since a re-acquired angle writes a block the old layout has no
room for. A resumed v7 angle rewrites its background in place, and the
engine checks the new block fits the room the file has before writing it —
anything else would shift every row behind it.

Two fixes made along the way:

  * QtScanController never accepted file_version, so every scan launched
    from the app raised TypeError at construction.
  * angle_status() left its cursor parked at the frontier, so every angle
    past it reported the frontier's own data_offset — which handed a resumed
    scan the same write position for several angles. Two recorded offsets in
    tests/golden/sras_expected.json are corrected accordingly.

The v6 goldens stay as parser fixtures; the writer is now locked against
bytes the test lays out from scan_format.md itself.

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

KNOWN_ISSUES.md

@@ -48,3 +48,19 @@ rig, check whether the camera apps run without the shim; if they do, delete
 `lib/ueye_loader.{c,so}`. Either way, record in SETUP.md where
 `libueye_api64.so.3.82` came from (IDS SDK version) and how the loader is
 meant to be used.
+
+## Per-angle background: trigger round trip mid-scan
+
+Every angle now captures its own background, so the scope switches from the
+scan-time logic-AND trigger back to the single-record edge trigger and
+returns to it once per angle (`core/scope_sras.py`:
+`configure_background_trigger` → `capture_background` →
+`configure_scan_trigger`). Before this, that transition happened once per
+scan, with the stage idle and nothing depending on how long it took.
+
+**Bench check:** run a multi-angle scan and watch the first row after each
+background. If frames go missing at the start of an angle, the 0.2 s settle
+in `configure_scan_trigger` is not enough for FastFrame to re-arm after an
+AVERAGE-mode sequence, and the row-packing warning ("N frames acquired, M
+expected") will say so in the log. Raise the settle rather than the ramp
+buffer — the stage geometry is not what changed.