#415 Store project_id in usearch metadata

Problem

Vector search over-fetches, then filters by loading ALL project symbol IDs into HashSet.

src/index/brain.rs:198-208

Store project_id in usearch index, filter during KNN search.

Eliminate large DB query per search. Save 10-20ms for large projects.

Meta-tracker. Sub-issue #415 is straightforward but low priority — project_id stored in vector cache already.