perf: parallel finite validation and scratch-based rank encode

[Fieldnote-Echo]

Summary

• assert_all_finite paid a full serial pass per add/search batch — measured 0.097s per GiB. Inputs ≥1M floats now split the sweep across the rayon pool (0.097s → 0.022s measured on a real-corpus GiB slice); the panic message is unchanged.
• RankQuant::add's parallel closure allocated a fresh ranks Vec per row; for_each_init now reuses per-worker scratch via rank_transform_into.

Measured (262,144 × 1024 real-corpus slice, best-of-3)

Path	before	after
RankQuant::add	0.302s	0.268s
SignBitmap::add	0.090s	0.062s
OrdinalIndex::add_2d (downstream)	0.489s	0.416s

Companion to ordinaldb perf/parallel-input-validation (same fix at the caller layer, preserving exact first-offender panics). Independent of #277/#278 — branches off main, touches only util.rs/quant.rs.

Full gate: 250 tests + no-default-features + clippy -D warnings green.

Part of the 1M-row release train (encode lane).

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Codex (@codex) review

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[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/util.rs	60.00%	2 Missing ⚠️

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/agentic_review

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Code Review by Qodo

🐞 Bugs (0) 📘 Rule violations (0) 📎 Requirement gaps (0)

Great, no issues found!

Qodo reviewed your code and found no material issues that require review

[Fieldnote-Echo]

Merged to main via #283's squash (bee2fad) — main's tree is byte-identical to the final collapse tip (verified: git rev-parse origin/main^{tree} == docs/release-hygiene^{tree}). Squash-merging broke the head-reachability that would have auto-closed this PR; closing manually. All review findings on this PR were remediated in-branch before the collapse; the commits and review trail remain linked here.