Have Lookup check the disk for variants held in memory too (closes #70)

Lookup again counts a variant as present only when its file exists.
Treating a variant held in memory as present saved one check of the
disk, but no test covered it, and GetVariant opens the file anyway.

Model: opus-5-5

TODO.md

@@ -33,12 +33,12 @@ P2: security: referer blacklist
   `Cache.metaCache` holds the content types of up to 10,000 variants in an LRU
   (`github.com/hashicorp/golang-lru/v2`), filled by `StoreVariant` and by
   `GetVariant` after it reads a `.meta` file, where a type `StoreVariant` added
-  meanwhile is kept over the one read; for a variant it holds, `Lookup`
-  skips the check of the disk and `GetVariant` skips the `.meta` read, still
-  opening the variant file and taking the size from it; eviction removes the
-  entry before deleting the files, and `GetVariant` removes it when the file
-  will not open; the cap is a constant, not a setting; the unused `variantMeta`
-  type is gone; `README.md` describes it.
+  meanwhile is kept over the one read; for a variant it holds, `GetVariant`
+  skips the `.meta` read, still opening the variant file and taking the size
+  from it; eviction removes the entry before deleting the files, and
+  `GetVariant` removes it when the file will not open; the cap is a constant,
+  not a setting; the unused `variantMeta` type is gone; `README.md` describes
+  it.
 - 2026-09-29 bound concurrent image processing and upstream fetches (closes
   #64): `max_concurrent_processing` (default the number of CPUs pixa can use)
   limits the images decoded and encoded at once, and `upstream_connections`