## Why
Cache presence and freshness checks deserialize full plugin records even when callers only need to decide whether a cached catalog is available.
## What changed
- Probe cache availability without deserializing plugin records, while still requiring a valid catalog envelope and a `plugins` array.
- Let `PreferFreshCache` reject stale catalogs from a 256-byte prefix when the header provides enough information; fall back to a full read otherwise.
- Limit discovery cache probes to the user and workspace scopes that need them, skipping the global cache probe.
- Serialize cached plugin slices directly without cloning them.
## Testing
Add coverage for malformed cache headers and payloads, reordered fields, stale-prefix rejection, and deferring plugin-record validation until a full catalog load.
GitOrigin-RevId: 1e5ec95917f38c84bba30dbd3bb8c2f91cc7e3a8
Charlie Marsh
a8bca62978