feat(core): parameterize quantizer bit-width (5/6-bit) — dominates residual mode on size AND speed #39

Motivation

The residual mode (#23, #38) buys recall at 2x storage (514→1028 B/vec) and +43% scan cost. Diagnostic (real EmbeddingGemma + faithful-port clustered set, release, aarch64) shows a plain higher-bit single-pass mode dominates it at these operating points:

Real EmbeddingGemma (n=2000+100, d=768, padded 1024)

Clustered adversarial (d=384, vecq-bench generator port)

6-bit plain matches or beats residual recall on BOTH datasets at 25% less storage and no second-pass penalty. 5-bit is the sweet spot for size-sensitive profiles (r@10 0.983 real at ~half residual's size). Residual keeps one niche: perfect recall@1 on real data.

Proposal

• Add quantizer width to the index config: 4-bit (default, unchanged), 5-bit, 6-bit.
• Lloyd-Max tables for 32/64 levels: precomputed constants (same offline procedure as 4-bit).
• Packing: bit-packing (5/6 bits/code) replacing nibble-packing for the non-4-bit paths; format header gains a width field (v1.5, readers accept v1–v4).
• Scoring: LUT gather like the 4-bit NEON path, wider tables (32/64 entries → two/four 16-byte table segments + select); bit-identity tests scalar vs NEON as usual.
• Keep residual as-is (opt-in); document the operating points honestly in BENCHMARK.md: 5-bit for max compression-per-recall, 6-bit for recall ≈ residual at less storage, residual for max recall@1.

Acceptance

• Bit-identity across scalar/NEON for all widths.
• recall@10 within noise of the diagnostic numbers above on the real dataset test (real_residual_validation.rs extended per width).
• Plain 4-bit path untouched (bit-identical baseline re-verified per merge policy).

Numbers reproducible by re-running the same procedure: Lloyd iteration on a dense N(0,1) grid at 32/64 levels, real RHDH rotation, exact-norm residual reference (post-#38 estimator).