Keep variant content types in memory for cache hits (closes #70)

Cache.metaCache was declared and never used, so every hit read and
parsed the variant's .meta file. It is now an LRU of up to 10,000
content types (hashicorp/golang-lru/v2), filled by StoreVariant and by
GetVariant after it reads a .meta file. For a variant it holds, Lookup
skips the disk check and GetVariant skips the .meta read; the variant
file is still opened and its size taken from it. Eviction removes the
entry before deleting the files, and GetVariant removes it when the
file will not open, so a missing variant is never served. The cap is a
constant, not a setting. README.md describes it.

Model: opus-5-5

README.md

@@ -86,7 +86,10 @@ prevent abuse, and allowlisted source hosts for open access.
 
 Multiple source paths may reference the same content blob; the
 database tracks references rather than using filesystem refcounting.
-In-process caching of request-to-output mappings targets 1-5k r/s.
+Toward a target of 1-5k r/s, pixa keeps in memory the content types of
+the 10,000 transformed images most recently cached or served, so a
+cache hit on one of them reads only the image file from disk and not
+the metadata file stored beside it.
 
 ### Routes