feat(brain): vecq-core 0.3.0 — keyed persistence, residual default, brain.vector_bits knob

[ajianaz]

What

Closes #547: vecq-core 0.2.0 → 0.3.0, and uses what it ships.

• Keyed persistence (upstream vecq#32 closed, format v1.3+): load_or_create_vecq now deserializes for real and keeps a healthy keyed index as-is. Legacy keyless files (pre-0.3.0), corrupt files, and dim mismatches rebuild dirty so the next cora index re-embeds. A dim guard is mandatory — add_keyed panics on wrong-dim vectors.
• brain.vector_bits knob (residual|4|5|6, lenient, default residual): 4-bit base codes + second-pass residual rescoring. A recall study on cora's own embeddings (real Rust symbol texts through the default hashing provider, ground-truth cosine) put residual at recall@10 0.930/0.917/0.863 at 1k/5k/13k symbols — ahead of plain 5-bit (0.880/0.850/0.840) at every scale, at 4-bit scan speed. A width change rebuilds the index once so the config takes effect; vecq_file_needs_rebuild is width-aware.
• Cross-project heal: the vector index is one shared file across projects; a rebuild now clears embed fingerprints for ALL projects (previously only the active one — other projects kept "embedded" fingerprints while their vectors were gone). The usearch dims-mismatch deletion path joins the same heal.
• BrainConfig gets a manual Default (derived Default produced empty strings, inconsistent with the serde defaults).

Why

#546's lazy-load made the search path backend-aware "until upstream lands key serialization" — this is that serialization. Together: the vecq signal is now fully alive in fresh processes, and index rebuilds stop being a per-run cost.

Testing

• New: keyed roundtrip survives reload, legacy keyless rebuild, corrupt rebuild, dim-mismatch rebuild, width-switch rebuild, roundtrip across all four widths, lenient vector_bits parsing, config merge/defaults.
• Full suite 947 pass (923 unit + 2 fresh-process integration from fix(brain): load vector index from disk on the search path #546 + 16 + 6), clippy -D warnings, fmt clean, cora review --staged pass (pre-commit hook).
• E2E with the real binary: brain.vector_bits: "6" config → indexed → .vecq header verified on disk (config → binary → bytes).

Closes #547