Have Lookup check the disk for variants held in memory too (closes #70)

Lookup again counts a variant as present only when its file exists.
Treating a variant held in memory as present saved one check of the
disk, but no test covered it, and GetVariant opens the file anyway.

Model: opus-5-5

internal/imgcache/cache.go

@@ -159,15 +159,13 @@ type LookupResult struct {
 	CacheStatus CacheStatus
 }
 
-// Lookup checks if a processed variant exists on disk: a variant held
-// in metaCache counts as present without a check of the disk. Hits
-// touch the variant's LRU timestamp; a disabled cache always misses.
+// Lookup checks if a processed variant exists on disk. Hits touch the
+// variant's LRU timestamp; a disabled cache always misses.
 func (c *Cache) Lookup(ctx context.Context, req *ImageRequest) (*LookupResult, error) {
 	cacheKey := CacheKey(req)
 
-	// Check memory, then variant storage - no DB needed for cache hits
-	if !c.disabled &&
-		(c.metaCache.Contains(cacheKey) || c.variants.Exists(cacheKey)) {
+	// Check variant storage directly - no DB needed for cache hits
+	if !c.disabled && c.variants.Exists(cacheKey) {
 		c.touchVariant(ctx, cacheKey)
 
 		return &LookupResult{