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>

core/scope_sras.py

@@ -51,11 +51,13 @@ def configure_channels(scope, profiles=None) -> None:
         scope.set_channel_bandwidth(ch, p.bandwidth_hz)
 
 
-def configure_acquisition(scope) -> int:
-    """Program the edge trigger and timebase; returns samples per frame.
+def configure_background_trigger(scope) -> None:
+    """Program the edge trigger used for a background capture.
 
-    Edge trigger on the rising edge of CH2 (laser pulse).  FastFrame stays
-    off here so the background capture runs as a single record.
+    Edge trigger on the rising edge of CH2 (laser pulse), FastFrame off so
+    the capture runs as a single record.  Every angle starts with a fresh
+    background, so the scan comes back here between angles from the
+    logic-AND trigger configure_scan_trigger leaves behind.
     """
     scope.write("TRIGger:A:TYPe EDGE")
     scope.set_trigger_source(2)
@@ -64,6 +66,11 @@ def configure_acquisition(scope) -> int:
     scope.set_trigger_mode("NORMAL")       # wait for trigger (don't auto-sweep)
     scope.set_acquire_mode("SAMPLE")
     scope.set_fastframe_state(False)
+
+
+def configure_acquisition(scope) -> int:
+    """Program the edge trigger and timebase; returns samples per frame."""
+    configure_background_trigger(scope)
     # Pin the transfer format instead of inheriting front-panel state — the
     # file header hardcodes bytes_per_sample=1, and a scope left on 2 bytes
     # would corrupt every frame written.