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

go.mod

@@ -14,6 +14,7 @@ require (
 	github.com/go-chi/httprate v0.16.0
 	github.com/gorilla/csrf v1.7.3
 	github.com/gorilla/securecookie v1.1.2
+	github.com/hashicorp/golang-lru/v2 v2.0.7
 	github.com/prometheus/client_golang v1.23.2
 	github.com/slok/go-http-metrics v0.13.0
 	github.com/spf13/cobra v1.10.2