Fix embedding builder sub-target instantiation and flow state_dict cloning

[cweniger]

Summary

• builder.py: add _instantiate_sub_targets() — recursively instantiates any constructor kwarg dict that has _target_ but no _input_ (e.g. whitener: {_target_: DiagonalWhitener, dim: 17280}) into a real module before passing to cls(**kwargs). Previously these arrived as raw dicts, causing the receiving module to call dict.update(tensor) instead of module.update(tensor).
• flow.py: _update_best_weights now calls load_state_dict(module.state_dict()) directly, matching GaussianFullCov. The prior {k: v.clone() ...} comprehension broke for modules using get_extra_state() to store non-tensor objects (e.g. DynamicSVD's eigenbasis dict).

Test plan

• Confirm DynamicSVD with whitener: {_target_: DiagonalWhitener, ...} in YAML config instantiates correctly
• Confirm best-weights snapshot works when embedding contains a DynamicSVD
• Existing embedding builder tests pass

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 10.38%. Comparing base (d08c81c) to head (e8d8ba9).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
falcon/embeddings/builder.py	0.00%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
- Coverage   10.42%   10.38%   -0.04%     
==========================================
  Files          30       30              
  Lines        3896     3910      +14     
==========================================
  Hits          406      406              
- Misses       3490     3504      +14     

Flag	Coverage Δ
unit	10.38% <0.00%> (-0.04%)	⬇️

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