0.6.0 staging: tech-debt & module refactoring (reduce monoliths)

[Fieldnote-Echo]

Staging / tracking issue for the 0.6.0 cycle — internal tech debt and module refactoring, with no public API breakage.

Goal: break large monolithic modules into smaller, cohesive units (the repo style targets ~200–400 lines/file, 800 max) and consolidate duplicated dispatch/kernel boilerplate — without changing behavior, the public surface, or the on-disk formats.

This is a living checklist; items are added/triaged as we go.

Candidate areas (to triage)

• Split the largest core modules into focused units (separate the index type / scoring kernels / eval path / persistence glue where a single file has grown past the size guideline).
• rank_io.rs: consider per-format submodules sharing the common header/validation helpers.
• Factor the repeated AVX-512 → AVX2 → scalar SIMD dispatch boilerplate into a shared helper.
• Give the experimental research modules (contingency / multi-bucket / constant-weight bitmap) a clearer internal structure.
• Reduce test-file monoliths similarly.

Out of scope

• Anything that changes the public API or on-disk write formats → tracked in the 0.8.0 re-architecture issue.