Fix confusing weight loading logging for legacy models

[gitttt-1234]

Summary

• Fixes confusing logging when loading backbone/head weights separately from legacy .h5 files
• Adds component-based filtering to only process relevant weights
• Warnings now only show for truly unexpected mismatches

Problem

When loading legacy TensorFlow/Keras weights into separate PyTorch components (backbone vs head), users saw:

1. Confusing counts like "22/26 loaded" instead of "22/22"
2. Many warnings about "No matching PyTorch parameter" for weights that belong to the other component (expected behavior, not an error)

Solution

Added a component parameter to filter legacy weights before mapping:

• component="backbone": Only processes encoder/decoder weights (excludes "Head" layers)
• component="head": Only processes head layer weights
• component=None: No filtering (default, for full model loading in inference)

Changes

sleap_nn/legacy_models.py

• Added filter_legacy_weights_by_component() helper function
• Added component parameter to map_legacy_to_pytorch_layers() and load_legacy_model_weights()
• Cleaned up unused imports

sleap_nn/training/lightning_modules.py

• Updated backbone loading to pass component="backbone"
• Updated head loading to pass component="head"

Result

Before:

Loading backbone weights from `model.h5` ...
No matching PyTorch parameter found for model_weights/CentroidConfmapsHead_0/...
No matching PyTorch parameter found for model_weights/OffsetRefinementHead_0/...
Successfully mapped 22/26 legacy weights to PyTorch parameters

After:

Loading backbone weights from `model.h5` ...
Filtered legacy weights for backbone: 22/26 weights
Successfully mapped 22/22 PyTorch parameters from legacy weights

Test plan

• Verified fix with test script showing correct logging output
• All existing test_legacy_models.py tests pass (35 passed, 4 skipped, 3 xfailed)
• CI tests

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.58%. Comparing base (ff91433) to head (54f5cd4).
⚠️ Report is 125 commits behind head on main.

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Flag	BASE (ff91433)	HEAD (54f5cd4)
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Additional details and impacted files

@@             Coverage Diff             @@
##             main     #439       +/-   ##
===========================================
- Coverage   95.28%   84.58%   -10.70%     
===========================================
  Files          49       74       +25     
  Lines        6765    10785     +4020     
===========================================
+ Hits         6446     9123     +2677     
- Misses        319     1662     +1343     

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