Remove idle host semaphores and delete .meta with its variant (closes #87)

Each upstream host's semaphore now counts the fetches holding or waiting
for one of its slots, and is removed from hostSems when the last of them
gives its slot back or stops waiting, so a long-running pixad no longer
keeps one semaphore per host it ever fetched from. The semLen test helper
reads hostSems directly, as getHostSemaphore now counts its caller.

VariantStorage.Delete removes the variant's .meta file too, a missing one
not being an error; DeleteWithMeta, which eviction called for that, is
gone.

Model: opus-5-5

internal/imgcache/eviction.go

@@ -283,7 +283,7 @@ func (c *Cache) evictVariant(ctx context.Context, cacheKey VariantKey) error {
 
 	c.metaCache.Remove(cacheKey)
 
-	err = c.variants.DeleteWithMeta(cacheKey)
+	err = c.variants.Delete(cacheKey)
 	if err != nil {
 		return err
 	}