#413 Lazy embedding -- defer to first brain query #416

Problem

brain::embed_project() runs as part of cora index, embedding ALL symbols even if cora brain is never used.

Location

src/index/mod.rs:273

Solution

1. Remove from index flow
2. Call lazily on first cora brain
3. Track embedding state per project
4. Only embed new/changed symbols (incremental)

Expected Impact

Index time: -50%. Brain first query slightly slower (~100ms one-time).