fix: avoid duplicate reaction when reconnecting a derived

When a derived is disconnected (its last reader is hidden), then its
dependency is changed and the reader is shown again, `reconnect`
unconditionally pushed the derived into the dependency's `reactions`
array. If it was already present, this left a duplicate entry that
`remove_reaction` only removes once, leaking the derived (and any
component it is owned by) after unmount.

Guard the push with a membership check so a derived is only ever
registered once per dependency.

.changeset/lazy-rivers-connect.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid duplicate reaction when reconnecting a derived after its reader is hidden

packages/svelte/src/internal/client/runtime.js

@@ -736,7 +736,10 @@ function reconnect(derived) {
 	if (derived.deps === null) return;
 
 	for (const dep of derived.deps) {
-		(dep.reactions ??= []).push(derived);
+		var reactions = (dep.reactions ??= []);
+		if (index_of.call(reactions, derived) === -1) {
+			reactions.push(derived);
+		}
 
 		if ((dep.f & DERIVED) !== 0 && (dep.f & CONNECTED) === 0) {
 			unfreeze_derived_effects(/** @type {Derived} */ (dep));