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, are handed without waiting to the goroutine that wrote late
counts, which now writes every count and access time 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. make bench
runs a parallel cache hit benchmark with Go on the host, outside make
check.

Model: opus-5-5

internal/imgcache/eviction.go

@@ -512,16 +512,12 @@ func (c *Cache) reconcileAccounting(ctx context.Context) error {
 	return c.reconcileUsageTotal(ctx)
 }
 
-// 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 {
+// 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 filepath.WalkDir(
-		baseDir,
+		c.variants.baseDir,
 		func(path string, entry fs.DirEntry, err error) error {
 			if ctx.Err() != nil {
 				return ctx.Err()
@@ -539,21 +535,11 @@ func (c *Cache) reconcileFiles(
 				return nil
 			}
 
-			return reconcileFile(path, entry, name)
+			return c.reconcileVariantFile(ctx, path, entry, imagerequest.VariantKey(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
@@ -710,10 +696,28 @@ 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 c.reconcileFiles(ctx, c.srcContent.baseDir,
-		func(path string, entry fs.DirEntry, name string) 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.reconcileSourceFile(ctx, path, entry, imagerequest.SourceKey(name))
-		})
+		},
+	)
 }
 
 // reconcileSourceFile checks one source blob file against its