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/eviction.go

@@ -318,6 +318,10 @@ func (c *Cache) evictSourceBlob(ctx context.Context, contentHash ContentHash) er
 		return fmt.Errorf("failed to commit eviction transaction: %w", err)
 	}
 
+	if c.evictSourceBlobTestHook != nil {
+		c.evictSourceBlobTestHook(contentHash)
+	}
+
 	// Only after the rows are gone may the files be removed.
 	for _, reference := range references {
 		if err := c.srcMetadata.Delete(reference.host, reference.pathHash); err != nil {