test: add failing test for evictSourceBlob unlink-vs-store TOCTOU

Adds an instrumentation seam (evictSourceBlobTestHook, fired after the
row-deletion transaction commits and before the content file is
unlinked) and a test that pauses eviction there while a concurrent
StoreSource for identical content bytes races it. Currently red: the
store completes immediately instead of being excluded, which is
exactly the window the review flagged between evictSourceBlob's commit
and its unlink.

internal/imgcache/cache.go

@@ -76,6 +76,19 @@ type Cache struct {
 
 	// In-memory cache of variant metadata (content type, size) to avoid reading .meta files
 	metaCache map[VariantKey]variantMeta
+
+	// contentLocks serializes StoreSource and evictSourceBlob per
+	// content hash, closing the race window between an eviction's row
+	// deletion and its file unlink against a concurrent store of
+	// identical content.
+	contentLocks *contentLock
+
+	// evictSourceBlobTestHook, when set, is invoked by evictSourceBlob
+	// after its row-deletion transaction commits and before the
+	// content file is unlinked. It exists solely so tests can
+	// deterministically pause inside that window to exercise
+	// concurrent stores against it; production code leaves it nil.
+	evictSourceBlobTestHook func(ContentHash)
 }
 
 // NewCache creates a new cache instance.
@@ -94,6 +107,7 @@ func NewCache(db *sql.DB, config CacheConfig) (*Cache, error) {
 		evictionStop:     make(chan struct{}),
 		evictionDone:     make(chan struct{}),
 		metaCache:        make(map[VariantKey]variantMeta),
+		contentLocks:     newContentLock(),
 	}
 
 	if c.disabled {