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_viewer.py

@@ -1127,7 +1127,7 @@ class ManualAlignmentDialog(QDialog):
         threshold = self.spin_mask_threshold_mv.value()
         fy, fx = self._downsample
         self._masks_small = {
-            a: compute._block_mean_2d((img >= threshold).astype(np.float32), fy, fx)
+            a: compute.block_mean_2d((img >= threshold).astype(np.float32), fy, fx)
             for a, img in self._dc4_mv.items()
         }
 
@@ -1143,7 +1143,7 @@ class ManualAlignmentDialog(QDialog):
         mask-threshold change, Auto De-rotate, a rotation nudge/edit of the
         active angle. NOT triggered by a translation-only nudge — see
         _refresh_active_preview_layer."""
-        dx_ref, dy_ref = compute._pixel_pitch_mm(self._sras, self._ref_angle_idx)
+        dx_ref, dy_ref = compute.pixel_pitch_mm(self._sras, self._ref_angle_idx)
         fy, fx = self._downsample
         pitch = (dx_ref * fx, dy_ref * fy)
         origin, shape = compute.canvas_for_params(
@@ -1288,7 +1288,7 @@ class ManualAlignmentDialog(QDialog):
         for a in range(self._sras.n_angles):
             if a == self._ref_angle_idx:
                 continue
-            self._angle_params[a].rotation_deg = sign * compute._nominal_delta_deg(
+            self._angle_params[a].rotation_deg = sign * compute.nominal_delta_deg(
                 self._sras, a, self._ref_angle_idx)
             n_changed += 1
         self._sync_active_spinboxes()
@@ -1364,7 +1364,7 @@ class ManualAlignmentDialog(QDialog):
                  f"{worst[1][1]})."]
         drifted = []
         for a, _note in rows:
-            nominal = compute._nominal_delta_deg(self._sras, a, self._ref_angle_idx)
+            nominal = compute.nominal_delta_deg(self._sras, a, self._ref_angle_idx)
             got = self._angle_params[a].rotation_deg
             dev = min(abs(got - nominal), abs(got + nominal))
             if dev > 1.0: