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>

tests/test_alignment.py

@@ -40,7 +40,7 @@ def mm_transform(sras: SrasFile, result, angle_idx: int) -> np.ndarray:
     per-angle pixel pitches; undoing both must leave something orthonormal, or
     the transform is smuggling in a scale or a shear.
     """
-    dx_a, dy_a = compute._pixel_pitch_mm(sras, angle_idx)
+    dx_a, dy_a = compute.pixel_pitch_mm(sras, angle_idx)
     A_out = np.array([[0.0, result.canvas_dx_mm], [result.canvas_dy_mm, 0.0]])
     D = np.array([[0.0, 1.0 / dy_a], [1.0 / dx_a, 0.0]])
     return np.linalg.inv(D) @ result.per_angle[angle_idx].matrix @ np.linalg.inv(A_out)
@@ -99,7 +99,7 @@ def test_stage_coordinates_are_not_consulted(rig):
     moved = SrasFile(str(rig.path))
     for a in range(1, moved.n_angles):
         moved.x_start_mm[a] += 13.5 * a
-        moved._y_pos_per_angle[a] = moved._y_pos_per_angle[a] - 9.25 * a
+        moved.y_pos_per_angle[a] = moved.y_pos_per_angle[a] - 9.25 * a
     moved_dc4 = dc4_images(moved)
     moved_fits = {a: compute.register_angle_to_reference(
                       moved, a, 0, moved_dc4, dc_threshold_mv=_THRESHOLD_MV)
@@ -118,7 +118,7 @@ def test_canvas_is_reference_grid_extended(rig):
     assert np.allclose(t0.offset, np.round(t0.offset)), \
         f"angle 0 does not land on whole canvas pixels: {t0.offset}"
     assert ((result.canvas_dx_mm, result.canvas_dy_mm)
-            == compute._pixel_pitch_mm(sras, 0)), \
+            == compute.pixel_pitch_mm(sras, 0)), \
         "canvas pitch is angle 0's own pitch"
 
     n_rows, n_cols = result.canvas_shape
@@ -169,7 +169,7 @@ def test_downsampled_preview_lands_with_full_res(rig):
         result.canvas_origin_mm, result.canvas_shape)
     fy, fx = 4, 16
     small = compute.reproject_mask(
-        sras, a, 0, compute._block_mean_2d(full_mask, fy, fx),
+        sras, a, 0, compute.block_mean_2d(full_mask, fy, fx),
         p.rotation_deg, p.shift_mm, (pitch[0] * fx, pitch[1] * fy),
         result.canvas_origin_mm,
         (result.canvas_shape[0] // fy, result.canvas_shape[1] // fx),