SnapshotCache

A content-addressed cache of serialized snapshots keyed by commit SHA and the running serialkompat version. Because a commit's source is immutable, a cached snapshot for a SHA can be reused without recomputing — the baseline never has to rebuild an unchanged ref twice (design §5). The tool version is part of the key (design §5: "baseline SHA + tool version") so an upgrade never reuses a baseline cached under an older serialkompat: each version reads and writes its own subdirectory, and entries of any other version — including the legacy root-level <sha>.snapshot layout — are never consulted.

Reads are validated and writes are atomic, so a corrupt or half-written cache entry (e.g. a CI job killed mid-write) is never trusted as a baseline — that would silently under-report changes (#16 "corrupt cache ⇒ refuse to run").