- import build_from_pretrained_kwargs inside _load_bge_m3 (it's a local import
  of _load_embedding_model, not module-level) — the NameError made every bge-m3
  load fall over so the model silently degraded to the dense-only path.
- cast the sparse/colbert heads to the base model's dtype (bf16) so the head
  matmul over the last hidden state doesn't dtype-mismatch.
- requires_grad_(False) on the heads (inference only) — no autograd graph per
  request.
- log the bge-m3 backend selection at load.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Mw2KQiswmD69T45fTfjKwW