[Feature] Bypass manifest lists on exact snapshot live-entry cache hits

Start with AppendOnlyFileStoreScanTest.TestSnapshotLiveManifestCachePath and the public orc/append_09.db/append_09 fixture, using snapshot 5 and bucket 0 to reproduce the cache-hit path. Review draft implementation #374 and its focused regression tests for manifest-cache access counts, changed predicates, eviction/rebuild, and retained file counts. Done means eligible exact hits skip manifest reads and decoding while misses rebuild completely and filters, snapshot identity, and metrics remain correct.

Search before asking

• I searched existing issues and found no report of this exact-hit planning overhead.

Motivation

Repeated scans of the same snapshot and bucket still read and decode the manifest lists before checking the snapshot live-manifest-entry cache. The cached live entries already contain the state needed for that bucket, so this work adds avoidable CPU and metadata access to repeated point-read planning.

To reproduce, use the public orc/append_09.db/append_09 test fixture with snapshot 5 and bucket 0, enable the snapshot live-manifest-entry cache, and reuse one cache across two scans. Instrument the MANIFEST cache accesses, as in AppendOnlyFileStoreScanTest.TestSnapshotLiveManifestCachePath. Before the proposed change, the second scan revisits the manifest-list path even though its exact snapshot/bucket live-entry lookup hits.

Expected: an eligible exact hit bypasses manifest-list reads and decoding, while each request still applies its own filters. A miss must rebuild the complete bucket. Snapshot selection and the existing snapshot-identity validation must remain intact.

Solution

Resolve the snapshot first and consult the existing snapshot live-entry cache before reading manifest lists. Retain the table file count in the transient cached payload so skipped-file metrics remain correct; report zero scanned manifests for an exact hit. Version the transient payload so incompatible entries fall back to rebuilding.

Draft implementation: #374. Regression tests check the manifest-cache access count, changed predicates, eviction/rebuild, and retained file counts. This change does not introduce a filesystem block cache or alter the persisted table format.

Anything else?

The identical patch passed 88 focused tests in a downstream C++17 Release build. Upstream CI for the draft still requires maintainer approval; no standalone end-to-end performance uplift is claimed for this individual optimization.

Are you willing to submit a PR?

• I am willing to submit a PR; #374 is available as a draft.