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>

tools/check_equivalence.py

@@ -47,8 +47,8 @@ def row_slice(sras: SrasFile, angle_idx: int, n_rows: int) -> SrasFile:
     view.n_rows[angle_idx] = n
     view.data = list(sras.data)
     view.data[angle_idx] = sras.data[angle_idx][:n]
-    view._y_pos_per_angle = list(sras._y_pos_per_angle)
-    view._y_pos_per_angle[angle_idx] = sras._y_pos_per_angle[angle_idx][:n]
+    view.y_pos_per_angle = list(sras.y_pos_per_angle)
+    view.y_pos_per_angle[angle_idx] = sras.y_pos_per_angle[angle_idx][:n]
     return view