Dedup format layer; public accessors replace private reach-throughs

- SrasFile: _parse_v6 now retains per-angle x_delta and the verbatim
  preamble/background byte spans, and gains public data_offset,
  y_pos_per_angle, and iter_angle_blocks() (which now owns the ragged
  block-offset walk used three separate places before).
- sras_edit_scans: the 70-line re-parse of the v6 header sections
  (_read_v6_sections/_reread_span/_v6_angle_offsets) collapses into a
  _write_v6 that consumes SrasFile directly — verified byte-identical
  round-trip on v6 int8/int16 and legacy files. Eight print-and-exit
  pairs become _die().
- tools/make_test_sras imports the struct layouts from sras_format and
  the rotation matrix from sras_compute instead of re-declaring them
  (byte assembly stays independent of the reader).
- sras_compute: block_mean_2d, pixel_pitch_mm, nominal_delta_deg made
  public (they were GUI-facing); registration_workers() and
  default_max_workers() wrap the remaining private reach-throughs from
  sras_workers.

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

sras_workers.py

@@ -253,7 +253,8 @@ class BatchCacheWorker(QObject):
         for path in paths:
             try:
                 err = cache_file(path, self._mode, self._apply_bg_sub,
-                                 compute.get_fft_backend(), compute._MAX_WORKERS)
+                                 compute.get_fft_backend(),
+                                 compute.default_max_workers())
             except Exception as exc:
                 err = str(exc)
             done += 1
@@ -382,7 +383,7 @@ class CrossCorrelateWorker(_PooledWorker):
         self._search_deg = search_deg
 
     def _plan(self) -> int:
-        return compute._registration_workers(self._sras, compute._DEFAULT_FINE_DIM)
+        return compute.registration_workers(self._sras)
 
     def _items(self):
         return self._angles