The integrity scan selected whole rows, so a cold pass paged in the
changes/detail payloads and the client fingerprint for up to
MAX_INTEGRITY_SCAN_ROWS rows the verifier never reads. Project only the hash
columns, and add a lightweight verifier so the scan no longer builds a full
display dict per row.
Repair the read cache so it is a latency shield, not a correctness shortcut:
a result computed within INTEGRITY_CACHE_TTL_SECONDS is served from the
per-Workspace cache (opening, refreshing and paging all land inside that
window and pay nothing), while a cache miss re-verifies the whole window. An
incremental scan that skips previously verified ids can never see an edit to
an already-cached row -- exactly the tampering this feature exists to expose.
Verified against a live 414-row log: 50 content edits and 7 re-signed links
are all detected, including an edit to a row verified on a previous pass.
Also fix two correctness gaps and one maintenance bug:
- age-based prune deleted rows without invalidating the cached prefix;
- the boundary baseline is now read only when the history exceeds the scan
window, which a bounded scan makes the rare case;
- the operation-log retention block had drifted outside the per-binding loop
in the maintenance task, so only the last discovered Workspace was ever
pruned while the others grew unbounded.
Tests: scan projection, verifier parity, TTL cache reuse, cache-miss
re-verification, edit to an already-verified row, hash mismatch, chain break
and prune invalidation.
