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

internal/imgcache/pendingcounts_internal_test.go

@@ -155,12 +155,11 @@ func TestService_Get_ReturnsByItsDeadlineWhileTheDatabaseIsBusy(t *testing.T) {
 	waitForCounters(t, svc.cache, cacheStatsCounters{missCount: 1})
 }
 
-// TestIncrementStats_ManyCountsLeaveOneWriteWaiting holds the database's one
+// TestIncrementStats_ManyCountsPastTheirDeadlineLeaveOneWriteWaiting holds the
-// connection while many misses are added to the pending counts, which the
+// database's one connection while many misses are counted past their
-// goroutine StartPendingCountWrites starts writes to the database. At most one
+// deadline. At most one write may then wait for the connection, and every
-// write may then wait for the connection, and every miss must reach the
+// miss must reach the database once it is free.
-// database once it is free.
+func TestIncrementStats_ManyCountsPastTheirDeadlineLeaveOneWriteWaiting(t *testing.T) {
-func TestIncrementStats_ManyCountsLeaveOneWriteWaiting(t *testing.T) {
 	t.Parallel()
 
 	cache, _ := newEvictionTestCache(t, 1<<30)
@@ -195,9 +194,9 @@ func TestIncrementStats_ManyCountsLeaveOneWriteWaiting(t *testing.T) {
 	waitForCounters(t, cache, cacheStatsCounters{missCount: misses})
 }
 
-// TestStats_IncludesPendingCounts adds hits and misses to the pending counts,
+// TestStats_IncludesPendingCounts counts hits and misses whose writes miss
-// with no goroutine started to write them, and checks that Stats adds them to
+// their deadline, and checks that Stats adds them to the database's counts
-// the database's counts before they are written.
+// before they are written.
 func TestStats_IncludesPendingCounts(t *testing.T) {
 	t.Parallel()