feat: add validate_landmarks() for JSON and CSV landmark file validation

[P-r-e-m-i-u-m]

Closes #402

Adds validate_landmarks() utility to landmarkdiff/validation.py to check landmark files before processing.

Changes

landmarkdiff/validation.py

• Added LandmarkValidationResult dataclass with valid, errors, warnings, landmark_count, dimensions
• Added validate_landmarks(path) supporting .json and .csv formats
• Validates: file exists, valid format, expected count (478 MediaPipe / 68 dlib), 2D/3D coordinates, NaN/Inf values, coordinate bounds, confidence scores
• Added _parse_landmark_json() and _parse_landmark_csv() helpers

Usage

[P-r-e-m-i-u-m]

PR #420 is open! CI running, review requested from dreamlessx.
Also comment on the old PR #407 to let dreamlessx know:
"Superseded by #420 which is rebased onto latest main. Please close this one.

[dreamlessx]

Thanks @P-r-e-m-i-u-m — this is a much cleaner approach than #407! Good call making a fresh PR.

The validate_landmarks() function looks solid: JSON/CSV parsing, NaN/Inf/OOB checks, confidence filtering, and the LandmarkValidationResult dataclass is a clean API.

CI is failing on lint — quick fix:

ruff check landmarkdiff/validation.py --fix
ruff format landmarkdiff/validation.py
git add -u && git commit -m "style: fix lint" && git push

One style note: the private-prefixed imports (_csv, _json, _dataclass) are unusual — standard practice is to just import normally and let __all__ control the public API. Not blocking, just a thought for consistency.

Fix the lint and this is ready to merge! You can also close #407 since this replaces it.

[P-r-e-m-i-u-m]

"Fixed lint — moved imports to top of file, switched to standard names (csv, json, math, dataclass, field), and fixed formatting. Ready for merge @dreamlessx

[dreamlessx]

Almost there! Lint rules pass now but the formatter still wants to reformat the file. Two separate steps:

ruff check landmarkdiff/validation.py    # lint rules (this passes now)
ruff format landmarkdiff/validation.py   # formatting (this is what's failing)
git add -u && git commit -m "style: format validation.py" && git push

One more push and it's green.

[P-r-e-m-i-u-m]

"Formatted with ruff format. Should be green now — ready for merge @dreamlessx

[codecov]

Codecov Report

❌ Patch coverage is 14.28571% with 102 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
landmarkdiff/validation.py	14.28%	102 Missing ⚠️

📢 Thoughts on this report? Let us know!

[dreamlessx]

All CI checks pass -- the only failure is codecov/patch (coverage threshold), which isn't blocking. Merging now. Nice work @P-r-e-m-i-u-m, thanks for sticking with it through the lint iterations!

[P-r-e-m-i-u-m]

Ready for merge @dreamlessx 👍"