A decode refusal kept serde's text, which quotes values read from the file
(`invalid type: string "..."`). The detail is now CultCache's own context and
record refusals, which name only a record's key and schema id, plus a fixed
description of the decoder failure's kind. rmp-serde reports no position, so
none is given. An unsupported header is echoed only in the shape
`cultcache.store.v<digits>`; any other is described by its length.
`is_absent` used `fs::metadata`, which follows links, and a read that found
nothing through a dangling link returned an empty store, so a writer then
renamed a regular file over the link and the store left its volume. Only
nothing at the path is absent now (`symlink_metadata`), and a read through a
dangling link is an io::Error.
Tests: canaries for a catalog and a legacy value and for unshaped headers,
the dangling link on every read with nothing written, and a
`cultcache.storeX` header that claims no format.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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