Make validate_mask faster

[lukas-lightly]

What has changed and why?

np.bincount() is faster than np.unique()

How has it been tested?

Existing tests

[lukas-lightly]

Benchmark report by Gemini:

Benchmark Results

  ┌────────┬───────────────────┬─────────────┐
  │ Branch │ Total Runtime (s) │ Improvement │
  ├────────┼───────────────────┼─────────────┤
  │ Main   │ 567.4s (~9.5 min) │ -           │
  │ PR 84  │ 323.0s (~5.4 min) │ ~43% Faster │
  └────────┴───────────────────┴─────────────┘

Summary

PR 84 demonstrates a significant performance gain, reducing the dataset ingestion time from over 9 minutes to approximately 5.5 minutes. Both runs processed the same number of samples and annotations, confirming the consistency of the benchmark.

• PR 84 Log: loaded samples=8034 labels=101 annotations=29546 metadata_updates=8034 elapsed_s=323.0
• Main Log: loaded samples=8034 labels=101 annotations=29546 metadata_updates=8034 elapsed_s=567.4

[lukas-lightly]

/review