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.go

@@ -512,12 +512,16 @@ func (c *Cache) reconcileAccounting(ctx context.Context) error {
 	return c.reconcileUsageTotal(ctx)
 }
 
-// reconcileVariantFiles walks the variant storage directory, checking
-// each variant file against its accounting row (reconcileVariantFile)
-// and sweeping stale temp files.
-func (c *Cache) reconcileVariantFiles(ctx context.Context) error {
+// reconcileFiles walks baseDir, one of the cache's storage directories,
+// sweeping stale temp files and passing each other file, with its name, to
+// reconcileFile. Once ctx is cancelled it stops at the next file and
+// returns ctx's error.
+func (c *Cache) reconcileFiles(
+	ctx context.Context, baseDir string,
+	reconcileFile func(path string, entry fs.DirEntry, name string) error,
+) error {
 	return filepath.WalkDir(
-		c.variants.baseDir,
+		baseDir,
 		func(path string, entry fs.DirEntry, err error) error {
 			if ctx.Err() != nil {
 				return ctx.Err()
@@ -535,11 +539,21 @@ func (c *Cache) reconcileVariantFiles(ctx context.Context) error {
 				return nil
 			}
 
-			return c.reconcileVariantFile(ctx, path, entry, imagerequest.VariantKey(name))
+			return reconcileFile(path, entry, name)
 		},
 	)
 }
 
+// reconcileVariantFiles walks the variant storage directory, checking
+// each variant file against its accounting row (reconcileVariantFile)
+// and sweeping stale temp files.
+func (c *Cache) reconcileVariantFiles(ctx context.Context) error {
+	return c.reconcileFiles(ctx, c.variants.baseDir,
+		func(path string, entry fs.DirEntry, name string) error {
+			return c.reconcileVariantFile(ctx, path, entry, imagerequest.VariantKey(name))
+		})
+}
+
 // reconcileVariantFile checks one variant file against its accounting
 // row. A file without a row is adopted into the accounting, unless it is
 // empty. A file that is empty, or not the size its row records, as a
@@ -696,28 +710,10 @@ func queryStringColumn[T ~string](
 // each blob file against its source_content row (reconcileSourceFile)
 // and sweeping stale temp files.
 func (c *Cache) reconcileSourceFiles(ctx context.Context) error {
-	return filepath.WalkDir(
-		c.srcContent.baseDir,
-		func(path string, entry fs.DirEntry, err error) error {
-			if ctx.Err() != nil {
-				return ctx.Err()
-			}
-
-			if err != nil || entry.IsDir() {
-				return err
-			}
-
-			name := entry.Name()
-
-			if strings.HasPrefix(name, tempFilePrefix) {
-				c.sweepStaleTempFile(path, entry)
-
-				return nil
-			}
-
+	return c.reconcileFiles(ctx, c.srcContent.baseDir,
+		func(path string, entry fs.DirEntry, name string) error {
 			return c.reconcileSourceFile(ctx, path, entry, imagerequest.SourceKey(name))
-		},
-	)
+		})
 }
 
 // reconcileSourceFile checks one source blob file against its