Expose constant-weight bitmap overlap and null-tail helpers

[Fieldnote-Echo]

Context

ordgraph-proto/src/bitmap.rs carries local ConstantWeightBitmap, PackedConstantWeightBitmap, top_group_overlap_vector, and BitmapNull helpers. The file itself notes that production scans should route through ordvec packed bitmap kernels.

This belongs in ordvec: it is ordinal bitmap encoding, overlap scoring, and finite null-model math.

Proposed API shape

• Stateless helper to build top-bucket/top-group bitmaps from RankQuant/rank bucket codes.
• Packed overlap helper for downstream evidence systems.
• Optional finite-null helpers for constant-weight bitmap overlap:
  • space size
  • exact overlap fiber count
  • upper-tail count/probability
• Clear separation between retrieval index APIs and stateless evidence helpers.

Acceptance criteria

• Parity tests against current ordgraph-proto/src/bitmap.rs.
• No dependency on ordgraph.
• Docs preserve the claim boundary: the uniform null is an in-model finite null, not a real-corpus guarantee.
• Downstream ordgraph can delete local bitmap/null code.

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Draft source: /home/ndspence/Downloads/ordgraph_extraction_issue_queue.md

[Fieldnote-Echo]

Reconciled during Ordinal Systems spinout tracking. Constant-weight bitmap overlap and finite null-tail helpers landed on main via merged PR #227; verified clean origin/main contains commit 0f40510. Downstream deletion of prototype copies remains bridge/harness migration work, but the ordvec-owned helper surface is complete.