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/make_test_sras.py

@@ -11,12 +11,19 @@ Usage:
 
 import argparse
 import struct
+import sys
 from pathlib import Path
 
 import numpy as np
 
-HDR_FMT_V6 = ">4sBHfffffffIdBB"
-GEO_FMT_V6 = ">ffIH"
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+# Single source of truth for the byte layout: the reader's own constants.
+# The byte *assembly* below stays independent, so a writer bug can't be
+# masked by a matching reader bug.
+from sras_format import HDR_FMT as HDR_FMT_LEGACY          # noqa: E402
+from sras_format import GEO_FMT_V6, HDR_FMT_V6             # noqa: E402
+from sras_compute import _rotation_matrix as _rot          # noqa: E402
 
 # Per-angle (n_rows, n_frames) — deliberately different per angle so ragged
 # geometry handling is actually exercised.
@@ -171,12 +178,6 @@ def _sample_shape_mv(u: np.ndarray, v: np.ndarray) -> np.ndarray:
     return img
 
 
-def _rot(theta_deg: float) -> np.ndarray:
-    t = np.radians(theta_deg)
-    c, s = np.cos(t), np.sin(t)
-    return np.array([[c, -s], [s, c]])
-
-
 def write_rotating(path: Path, n_angles: int = 5, samples_per_frame: int = 4,
                    seed: int = 0) -> dict:
     """Write a v6 file whose CH4 DC image is one sample seen at n_angles known
@@ -260,7 +261,6 @@ def write_rotating(path: Path, n_angles: int = 5, samples_per_frame: int = 4,
             "y_starts": y_starts, "dx_mm": _ROT_DX_MM, "dy_mm": _ROT_DY_MM}
 
 
-HDR_FMT_LEGACY = ">4sBHHffffIIdBB"
 
 
 def write_legacy(path: Path, version: int = 4, n_angles: int = 2,