Optional parallel feature (make rayon opt-out)

[Fieldnote-Echo]

Summary

Add a default-on parallel feature. When disabled, swap the par_chunks(_mut) / par_iter calls (in rank.rs, quant.rs, bitmap.rs, sign_bitmap.rs, fastscan.rs, multi_bucket.rs) for a single-threaded path, so callers in single-threaded WASM or thread-pool-averse environments can drop the rayon dependency.

⚠️ Scope correction

The external review framed this as "rayon is the only thing blocking no_std." That's not accurate. Every index type's write / load uses std::fs / std::io / std::path (e.g. src/bitmap.rs:408, src/rank.rs:506, src/sign_bitmap.rs:224). Full no_std would additionally require gating the entire persistence surface behind a std feature and auditing the rest of the crate.

This issue is scoped to making rayon optional only. A true no_std + alloc build is a separate, larger design discussion (and intersects the ROADMAP edge/WASM goals — worth its own issue if we want to pursue it).

Acceptance

• parallel feature (on by default); disabling it builds + tests green with no rayon in the tree
• Serial path produces identical results (determinism preserved)

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Filed from an external technical review (May 2026); scope corrected and verified against v0.2.0.