Serve a cache hit with no database read or write (closes #309)

A hit opens the variant file and nothing else: its content type comes
from the file's first bytes, and a file in no output format is a miss.
Its access time and hit count, and the access time of a cached source a
miss uses, go without waiting to the goroutine that wrote late counts,
which now writes them all once a second; each kind of access time waits
in a channel of 10,000 that drops the rest. checkNegativeCache leaves
expired rows to CleanExpired. The .meta files, metaCache, Lookup and
golang-lru go, with their tests. Dropping the .meta skip made the
variant and source reconciliation walks identical, so they share one.
make bench runs a parallel cache hit benchmark on the host, outside
make check.

Model: opus-5-5

TODO.md

@@ -30,6 +30,23 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-21 a cache hit makes no database call (closes #309): `Service.Get`
+  opens the variant file with `Cache.GetVariant` and nothing else, with no
+  existence check first. The content type comes from the file's first bytes; a
+  file whose first bytes are not those of an output format, SVG included, is a
+  miss, and the variant is made again. A hit hands its access time to a channel
+  that holds up to 10,000 and drops the rest, and its hit count to the pending
+  counts, without waiting; every count is now a pending count. A miss that finds
+  a cached source hands that source's access time to a second such channel. The
+  goroutine `StartPendingCountWrites` starts writes, once a second, the access
+  times in one transaction and the counts in one `UPDATE`;
+  `StopPendingCountWrites` writes what is left once at shutdown and logs
+  separately the access times and the counts it could not write.
+  `checkNegativeCache` no longer deletes an expired row; `CleanExpired` in the
+  eviction goroutine does. The `.meta` files, `VariantMeta`, `LoadWithMeta`,
+  `variantContentTypeFromSidecar`, `metaCache`, `Lookup` and `golang-lru` are
+  gone. `make bench` (`script/bench`) runs `BenchmarkHandleImage_CacheHits` with
+  Go on the host after `script/bootstrap --cgo`, outside `make check`.
 - 2026-10-21 a source is stored the way a variant is (closes #308): one
   `source_content` row and one file per source URL, the file named by the
   source's `SourceKey`, the SHA-256 of its host, path and query made as the