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/eviction_internal_test.go

@@ -469,15 +469,6 @@ func assertDisabledCacheReadsAlwaysMiss(
 
 	ctx := t.Context()
 
-	lookup, err := cache.Lookup(ctx, req)
-	if err != nil {
-		t.Fatalf("Lookup on disabled cache failed: %v", err)
-	}
-
-	if lookup.Hit {
-		t.Error("Lookup on disabled cache must always miss")
-	}
-
 	key, srcType, err := cache.LookupSource(ctx, req)
 	if err != nil {
 		t.Fatalf("LookupSource on disabled cache failed: %v", err)
@@ -616,7 +607,7 @@ func TestEvictionRunsOnPeriodicSchedule(t *testing.T) {
 	for i, key := range keys {
 		content := bytes.Repeat([]byte{fills[i]}, 1000)
 
-		_, err = cache.variants.Store(key, bytes.NewReader(content), "image/webp")
+		_, err = cache.variants.Store(key, bytes.NewReader(content))
 		if err != nil {
 			t.Fatalf("failed to store variant file: %v", err)
 		}
@@ -668,7 +659,7 @@ func TestStartEvictionReconcilesAccountingWithDisk(t *testing.T) {
 	untracked := bytes.Repeat([]byte{0x31}, 1000)
 
 	_, err := cache.variants.Store(
-		testVariantKeyOne, bytes.NewReader(untracked), "image/webp",
+		testVariantKeyOne, bytes.NewReader(untracked),
 	)
 	if err != nil {
 		t.Fatalf("failed to store untracked variant file: %v", err)
@@ -770,7 +761,7 @@ func TestPeriodicReconciliationAdoptsFileThatAppearsAfterStartup(t *testing.T) {
 	untracked := bytes.Repeat([]byte{0x41}, 900)
 
 	_, err = cache.variants.Store(
-		"aabbccdd0099", bytes.NewReader(untracked), "image/webp",
+		"aabbccdd0099", bytes.NewReader(untracked),
 	)
 	if err != nil {
 		t.Fatalf("failed to store untracked variant file: %v", err)