feat(brain): runtime embedding config + incremental per-symbol embedding

[ajianaz]

What

Add runtime embedding backend selection via .cora.yaml config and incremental per-symbol embedding to skip unchanged symbols during re-index.

Why

Previously, embedding backend was selected at compile time only (#[cfg(feature = "pretrained-embed")]). Users could not switch between hashing 256d and pretrained 768d without recompiling. Additionally, embed_project() re-embedded all symbols on every index run, even when only 1 file changed — wasting CPU on large projects.

Closes #499

How

BrainConfig (src/config/schema.rs):

• New brain.embedding field in .cora.yaml with values: auto (default), hashing, pretrained
• Invalid values fall back to auto with a warning
• BrainEmbeddingMode enum with Display + FromStr impls

Runtime dispatch (src/embed/mod.rs):

• New Backend enum (Hashing | Pretrained) with dims() and provider_name()
• resolve_backend() reads config string, caches via OnceLock
• embed_code_dispatch() checks ACTIVE_BACKEND at runtime, falls back to compile-time default if unset
• Graceful fallback: brain.embedding=pretrained without feature flag → warning + hashing

Incremental embedding (src/index/brain.rs):

• Migration v7 adds embed_fingerprint TEXT column to symbols table
• embed_project() computes DefaultHasher fingerprint from name + signature
• Only symbols whose fingerprint changed are re-embedded
• Fingerprints updated after successful embedding
• Vector index retains all symbol IDs for search (only embeddings are incremental)

Wiring (src/main.rs, src/commands/watch.rs):

• cora index, cora brain, cora watch all call resolve_backend() before embedding

Testing

• cargo test --features tree-sitter passes (881 + 16 + 6 = 903 tests, 0 failures)
• cargo fmt --all -- --check passes
• cargo clippy --all-targets --features tree-sitter -- -D warnings passes
• cargo build --release --features tree-sitter passes (hashing-only mode)
• cargo build --release --features tree-sitter,pretrained-embed passes (dual mode)
• Manual smoke-test: cora index on uteke (1106 symbols, 147 files)
  • Full index: 1106 symbols embedded ✅
  • No-change re-index: 147 files skipped, 0 symbols re-embedded ✅
  • Touch 1 file: 146 skipped, 1 indexed, only 2 symbols re-embedded ✅
  • cora brain "memory recall" returns 8 results with correct FTS5 + vector signals ✅
  • Both hashing-only and pretrained builds tested ✅

Related Issues

Closes #499

Checklist

• Branch name follows convention (feat/brain-runtime-config-499)
• Branch is from develop
• Commit messages follow Conventional Commits
• No secrets or credentials committed
• One logical change per PR (no mixed concerns)