testocache: harden persistent cache and add namespaces

[dmedovich]

TL;DR

Closes #25

This PR hardens the file-backed cache behavior and adds scoped cache namespaces for plugins sharing the
same cache directory.

Changes

• write cache entries through a temporary file followed by os.Rename;
• validate the stored key before returning or removing an entry;
• treat malformed and mismatched entries as missing;
• ignore foreign and non-regular files in Keys;
• preserve an existing .gitignore;
• return ErrNotFound consistently for missing entries;
• add testocache.Namespace for isolated keyspaces;
• document the cache behavior and add regression tests.

The existing package-level API and FNV-based cache filenames remain compatible.

Testing

• go test ./...
• go test -race ./testocache
• golangci-lint run --tests=false --timeout=5m
• git diff --check

[metafates]

Thank you, cool idea with the namespaces! Looks good overall, left some comments.

Also, thank you for the #23, will get it to a bit later

[metafates]

I've slightly improved atomic file writes and reused fnv hashing for namespaces instead of sha256