fix: avoid extra work in mark_reactions (#12921)

* fix: avoid extra work in mark_reactions

* Update packages/svelte/src/internal/client/reactivity/sources.js

Co-authored-by: Rich Harris <rich.harris@vercel.com>

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/flat-moose-arrive.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid extra work in mark_reactions

packages/svelte/src/internal/client/reactivity/sources.js

@@ -145,8 +145,8 @@ function mark_reactions(signal, status) {
 		var reaction = reactions[i];
 		var flags = reaction.f;
 
-		// Skip any effects that are already dirty
-		if ((flags & DIRTY) !== 0) continue;
+		// If a reaction is already dirty, skip it (but always mark unowned deriveds)
+		if ((flags & (CLEAN | UNOWNED)) === 0) continue;
 
 		// In legacy mode, skip the current effect to prevent infinite loops
 		if (!runes && reaction === current_effect) continue;