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 check landmark files before processing.

Changes

landmarkdiff/validation.py

• Added validate_landmarks(path) function supporting .json and .csv formats
• Added ValidationResult dataclass with valid, errors, warnings, landmark_count, dimensions
• Validates:
  • File exists and is readable
  • Valid JSON/CSV format with clear parse errors
  • Expected landmark count (478 MediaPipe, 68 dlib)
  • 2D or 3D coordinates
  • No NaN or infinite values
  • Coordinate bounds: [0, 1] normalized or [0, image_size] pixel
  • Confidence scores below threshold (if present)
• JSON schema: {"landmarks": [[x, y, z], ...], "confidence": [...]} or bare list
• CSV schema: x,y or x,y,z columns with optional header row

Usage

from landmarkdiff.validation import validate_landmarks

result = validate_landmarks("face.json")
if not result.valid:
    print(result)  # shows all errors clearly

[dreamlessx]

Nice implementation @P-r-e-m-i-u-m — the validate_landmarks() function is thorough with good JSON/CSV parsing, bounds checking, and confidence filtering.

This PR has a merge conflict though — likely from the other PRs that just merged. Can you rebase on latest main?

git fetch origin
git rebase origin/main
# resolve any conflicts
git push --force-with-lease

Once the conflict is resolved and CI passes, I'll merge it.

[dreamlessx]

Closing — superseded by #420 which is a clean fresh PR. Thanks!