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warn when a temporal-capable store has no readable root coverage-toc section (pruning degraded) #47

Description

@espg

Espg-directed (in-session 2026-08-18), the reader-side half of englacial/zagg#488. Refs #45 (the coverage_toc/when= surface this lands with) and zagg §10 (zagg-coverage-toc/1).

The situation to detect

A store whose leaves carry §8.3 temporal companions but whose root coverage.moc has no readable §10 section — either absent (clobbered by a pre-section producer; zagg's writer rebuilds the envelope from keys it knows) or at an unknown future revision (the strict gate reads it as absent). Both are legal states: the sidecar is a regenerable cache, and absence is never a refusal.

The behavior

When a when=-carrying query (or an explicit coverage_toc call) meets this state, warn once: temporal pruning is disabled, every spatial candidate will be opened (slow, correct), and the remedy is re-running zagg's refresh_root_coverage on the store. The warning should distinguish "no section" from "section at revision N, this reader knows ≤M" — the second case's remedy is upgrading the reader, not re-walking the store.

Not an error, not a refusal — a performance nudge with the exact fix named. Blocked on the #45 surface landing (and zagg#481 for a store to point at).

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