[codex] Validate plugin output before backprojection

[den-sq]

Summary

• Add a TIFF preflight for plugin segmentation outputs before backprojection starts.
• Validate straightened-volume D/V/U shape against the matching slice rects and reject unsupported mask dtypes with distinct messages.
• Fix directory-of-TIFF preflight/memmap handling by using the first sorted TIFF file rather than treating the sorted list as an iterator.
• Document plugin output shape, dtype, and compression expectations on the backprojection guide.
• Stabilize an existing float normalization assertion in test_files.py with assert_allclose.

Verification

PYTHONPATH=python /home/dnorthover/miniconda3/envs/lftomo/bin/python -m pytest python/test/helpers/test_files.py
PYTHONPATH=python /home/dnorthover/miniconda3/envs/lftomo/bin/python -m py_compile python/ouroboros/helpers/files.py python/ouroboros/pipeline/backproject_pipeline.py python/test/helpers/test_files.py

The requested combined helper command could not collect python/test/helpers/test_slice.py in this environment because neither system Python nor the lftomo env has cloudvolume installed:

ModuleNotFoundError: No module named 'cloudvolume'

Refs #80.

[tavateva]

Draft Promotion Review ❌ BLOCKED

Reviewed against Draft Promotion Criteria.

Criterion Assessment

1. CI and mergeability: ⚠️ AMBIGUOUS → FAIL — Mergeable, but 0 CI checks
2. Issue Acceptance Criteria: ❌ INCOMPLETE — Issue verification requires both test_slice.py and test_files.py; only test_files.py ran
3. PR body Test Plan checkboxes: ⚠️ INCOMPLETE — test_slice.py not run
4. Tests run: ❌ INCOMPLETE — Missing cloudvolume blocked test_slice.py execution
5. Scope: ✅ PASS
6. Regressions: ⚠️ CANNOT VERIFY — Partial test coverage
7. Judgement calls: ⚠️ CONCERNING — Dependency issue explained but not resolved

VERDICT: ❌ BLOCKED

Blocking Concerns

• Issue AUTO-5: Back-Project Plugin Output Through Ouroboros #80 verification section explicitly requires: pytest python/test/helpers/test_slice.py python/test/helpers/test_files.py
• Only test_files.py executed; test_slice.py skipped due to missing cloudvolume dependency
• Per criterion Load slice options correctly for backprojection cli. #2: "Acceptance Criteria that require live verification must be carried out AND their result reported — not deferred"

Remediation

Install cloudvolume, run test_slice.py, and report results in PR body before promoting from draft.