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>

README.md

@@ -12,10 +12,12 @@ scanengine-3 is a unified platform for scanning acoustic microscopy and precisio
 - **Laser Systems**: Helios pulsed laser and Genesis CW laser control
 - **Data Acquisition**: Tektronix oscilloscope integration with fast-frame support
 - **Scan Planning**: Automated raster scan generation and execution
+- **Per-Angle Background**: every angle opens with its own background
+  capture (Genesis off, Helios on), stored ahead of that angle's data
 - **Angle Inspection**: Park the rig at random points across a plan's angles
   to check the SAW response on the scope before committing to a long scan
 - **SAW Quality Check**: Acquire one row per angle — the row-wise middle of
-  the ROI — as a v10 `.sras`, then compare every angle's SAW frequency on one
+  the ROI — as a v11 `.sras`, then compare every angle's SAW frequency on one
   graph to judge the alignment before a full run
 - **Real-time Monitoring**: Live status updates and progress tracking
 
@@ -65,7 +67,7 @@ scanengine-3/
 │   ├── angle_inspect.py           # AngleInspector — park on a point per angle
 │   ├── saw_check.py               # Middle-row SAW check: plan + alignment read-out
 │   ├── rotation.py                # GR rotation axis settings + moves
-│   ├── sras_format.py             # v6/v10 .sras writer/reader (memory-mapped)
+│   ├── sras_format.py             # v7/v11 .sras writer, v6/v10 reader (mmap)
 │   ├── sras_analysis.py           # Image reducers + SAW matched filter
 │   └── config.py                  # ScanDefaults ⇄ aui_defaults.json
 │
@@ -98,7 +100,7 @@ scanengine-3/
 ├── sc3-aui-*.ui                   # Qt Designer files loaded at runtime
 │
 ├── tests/                         # pytest suite
-│   ├── golden/                    # v6 .sras + geometry fixtures
+│   ├── golden/                    # legacy v6 .sras + geometry fixtures
 │   ├── fakes.py                   # Recording fake stage/scope/rotator
 │   └── test_*.py
 │
@@ -220,7 +222,7 @@ print(f"wrote {result.rows_written} rows to {result.path}")
 ### Running a SAW quality check
 
 Same engine, same hardware sequence — the plan is reduced to one row per
-angle and the result is tagged v10 so the viewer knows it is a check rather
+angle and the result is tagged v11 so the viewer knows it is a check rather
 than a scan cut short:
 
 ```python