Delete / tombstone support via a sidecar skip-bitmap

[Fieldnote-Echo]

Summary

All four index types are append-only. Production corpora churn — documents are deleted, embeddings are recomputed as models update. A minimal tombstone facility would cover the common case without a full rebuild.

Ask

• delete(doc_id: u64) sets a bit in a sidecar tombstone bitmap.
• The scan path masks tombstoned doc IDs before they enter TopK.
• The sidecar is separate from the existing on-disk storage, so no .tvr* format change.

⚠️ Direction note (maintainer call)

ROADMAP.md lists "persistence and transaction machinery" as an explicit non-goal ("ordvec will not grow into a vector database"). A read-time skip-bitmap is arguably within scope — it's not compaction, MVCC, or transactions — but the framing needs a maintainer decision:

• Do tombstones persist alongside the index, or stay caller-managed (passed into search)?
• Explicitly defer compaction / space reclamation (that's the database territory the ROADMAP rules out).

If the answer is "caller-managed skip set passed to search," that may be a smaller, more on-philosophy first step than an owned mutable tombstone bitmap.

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Filed from an external technical review (May 2026); verified append-only against v0.2.0.