Add parallel image caching for faster dataset preparation (#323)

[gitttt-1234]

Summary

This PR implements parallel image caching to significantly speed up dataset preparation for large datasets. Using ThreadPoolExecutor with thread-local video copies achieves 3-4x speedup for datasets with 50+ frames.

• Add ParallelCacheFiller class for parallel I/O operations using thread-local video copies
• Add parallel_caching and cache_workers configuration options to DataConfig
• Update BaseDataset and all dataset subclasses to support parallel caching
• Add comprehensive tests for parallel caching with both HDF5Video and MediaVideo backends

Benchmark Results

Dataset Size	Sequential	4 Workers	Speedup
50 frames	0.71s	0.19s	3.8x
100 frames	1.51s	0.36s	4.2x

Key Implementation Details

• Uses thread-local video copies via deepcopy() for thread safety (pattern from providers.py)
• Automatically falls back to sequential caching for small datasets (<20 frames) where overhead exceeds benefit
• Lock-protected dictionary updates for memory caching
• Progress callback support for UI integration

Configuration Options

DataConfig(
    parallel_caching=True,  # Enable parallel caching (default: True)
    cache_workers=4,        # Number of workers (default: 0 = auto)
)

Test plan

• All existing tests pass
• New tests cover parallel caching with HDF5Video backend
• New tests cover parallel caching with MediaVideo backend
• New tests verify configuration options work correctly
• New tests verify thread safety with thread-local video copies
• Linting passes (black + ruff)
• Manually verify in windows / linux / mac

Closes #323

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.51%. Comparing base (ff91433) to head (6878a1a).
⚠️ Report is 125 commits behind head on main.

Files with missing lines	Patch %	Lines
sleap_nn/data/custom_datasets.py	79.62%	22 Missing ⚠️

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Flag	BASE (ff91433)	HEAD (6878a1a)
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Additional details and impacted files

@@             Coverage Diff             @@
##             main     #432       +/-   ##
===========================================
- Coverage   95.28%   84.51%   -10.77%     
===========================================
  Files          49       74       +25     
  Lines        6765    10875     +4110     
===========================================
+ Hits         6446     9191     +2745     
- Misses        319     1684     +1365     

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