feat: harden CLIP image RAG

[raoabinav]

Evaluator Summary

• Abi hardened LEANN's CLIP image retrieval path so local image corpora can be indexed with deterministic discovery, stable IDs, rich metadata, and text-to-image search.
• Design choice: image passages use stable image-<hash> identifiers and store file/path/size/dimension/model metadata, which makes rebuilds reproducible and result inspection reviewer-friendly.
• The implementation builds from precomputed CLIP embedding arrays and disables query-time recomputation for image vectors, because stored text cannot recreate the original pixels.
• Quality bar: 6 targeted fake-encoder tests avoid model downloads while covering discovery, metadata, stable IDs, and retrieval wiring, plus lint, format, type, lockfile, and whitespace checks.

[raoabinav]

Parking this PR for now to keep the review queue focused on #347. I’ll revisit/reopen this work later as a smaller clean PR once the content-hash PR lands.