Convert test scripts to pytest; extend the equivalence harness

- pyproject.toml replaces sras_viewer_requirements.txt (same pins) and
  adds a dev extra with pytest.
- tools/test_refactor.py, test_alignment.py, test_gui.py become
  tests/test_compute.py, tests/test_alignment.py, tests/test_gui.py with
  assertions preserved verbatim. test_gui.py stays one ordered
  integration sequence over a shared module-scoped window.
- check_equivalence.py: drop the dead pre-refactor monolith shim (and the
  _compute_angle_alignment alias it consumed), extend the pad sweep to
  (1, 2, 4, 8, 40), add legacy-v4 and big-endian int16 legs (new bps=2
  option in make_test_sras) so the padded FFT path and the >i2 memmap
  path are in the baseline before the FFT rewrite.

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

sras_compute.py

@@ -1188,10 +1188,6 @@ def compute_angle_alignment(sras: SrasFile, ref_angle_idx: int,
     return result
 
 
-# Back-compat alias for the pre-split private name (used by tooling).
-_compute_angle_alignment = compute_angle_alignment
-
-
 # ---------------------------------------------------------------------------
 # Manual alignment (Fusion menu -> Manual Alignment... dialog)
 #