Preserve cached hits from the youngest mature split

[PSeitz]

Summary

Exact query repeats can lose their cached pruning threshold when the newest splits merge away. Older splits often have only count-only cache entries, so the next refresh warms them up again before it can discard their hits.

Preserve a cached hit list from the youngest mature split, which is no longer eligible for merging. Its hits can establish a threshold on later refreshes and allow older count-only entries to be reused before warmup.

Changes

• Send split maturity from root to leaf through SplitIdAndFooterOffsets.is_mature.
• For timestamp-descending searches with hits and caching enabled, exempt the youngest fully time-covered mature split from top-hit pruning, including upfront match-all pruning and pruning after warmup.
• Add regression coverage for replacement splits, sliding windows, sparse/zero matches, boundary-crossing splits, count-only requests, and disabled caching.

Only one mature split is protected. If it has fewer than start_offset + max_hits matches, its cached hits alone cannot establish a threshold; older splits may still need searching. Collecting its hits adds work when it would otherwise have been pruned.

The protocol field is additive and defaults to false for older requests. No migration is required.

[chatgpt-codex-connector]

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-20T08:06:30.823459Z	f0b4f52	New commits

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[chatgpt-codex-connector]

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Reviewed commit: 23a1bf89b2

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[chatgpt-codex-connector]

Document the new split-maturity protocol

is_mature is a new root-to-leaf protobuf field that changes which split searches are exempted from pruning, but this patch leaves the protocol/cache documentation unchanged (docs/overview/concepts/querying.md:96 only gives a generic description of partial-result caching). Without documenting the false compatibility default and the timestamp-descending mature-split scope, operators and future protocol maintainers cannot reason about rolling upgrades or the added search work; add the corresponding protocol/cache documentation.

AGENTS.md reference: AGENTS.md:L23-L24

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