feat: allow users to pass custom timestamps to from_numpy

[muvva-krishna]

Closes #473

Allow users to pass a timestamps array to from_numpy() instead of
deriving time coordinates from fps.

Changes:

• Add timestamps field to _BaseDatasetInputs in validators/datasets.py
• Add length validation in ValidPosesInputs.to_dataset() before building
  the xarray Dataset (raises ValueError if length mismatches n_frames)
• Thread timestamps through load_poses.from_numpy() into ValidPosesInputs

Tests:

• test_from_numpy_with_custom_timestamps: verifies irregular timestamps
  are set correctly as the time coordinate
• test_from_numpy_timestamps_wrong_length_raises: verifies ValueError
  is raised when timestamp length does not match number of frames

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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