Add Debug impls for the public index types (params-only)

[Fieldnote-Echo]

Summary

None of the public index types — Rank, RankQuant, Bitmap, SignBitmap — nor SearchResults expose a Debug impl. The only Debug in src/ is on the internal SimdTier enum. Downstream users can't inspect an index at runtime without querying individual fields.

Ask

Add concise hand-written Debug impls (not #[derive]) that print the shape parameters only, e.g.:

RankQuant { dim: 1024, bits: 2, n_vectors: 50_000 }
Bitmap { dim: 256, n_top: 64, n_vectors: 50_000 }

Do not dump the packed buffers — a derived Debug on these would print megabytes of bytes. Params-only (dim/bits/n_top/n_vectors, maybe bytes-per-doc) keeps it useful in a pipeline log.

Acceptance

• Debug for Rank, RankQuant, Bitmap, SignBitmap, SearchResults
• Output is parameter summary, never the storage vectors
• {:?} smoke test per type

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Filed from an external technical review (May 2026); verified against v0.2.0 (only SimdTier derives Debug today).