fix: prevent batch unlinking twice (#18303)

Merged #18298 a bit too soon - there's still a situation which can
corrput the linked list: If we merge one batch into another, it will
unlink the batch immediately. But `discard` also calls it, running the
unlink logic again, which can corrupt the linked list.

Sketch:

1. Batch A is pending.
2. Batch B starts later, intersects A, resolves first, and merges into
A.
3. B is unlinked immediately, but `A.oncommit(() => B.discard())`
remains.
4. Independent batch C is created while A is still pending and is linked
after A.
5. A commits, calls `B.discard()`, and B's stale `#prev`/`#next` can set
`A.#next = null` / `last_batch = A`, disconnecting C.

Not able to produce a failing test from it but it's definitely a fix we
need to make.

Also moved `#link` into a constructor, because it is (and should be)
used only once.

Also made the action after an error `discard` instead of just `#unlink`
because this batch is done for and e.g. pending `settled` should
resolve, too.

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

@@ -214,6 +214,18 @@ export class Batch {
 
 	#decrement_queued = false;
 
+	constructor() {
+		// link batch
+		if (last_batch === null) {
+			first_batch = last_batch = this;
+		} else {
+			last_batch.#next = this;
+			this.#prev = last_batch;
+		}
+
+		last_batch = this;
+	}
+
 	#is_deferred() {
 		if (this.is_fork) return true;
 
@@ -327,11 +339,11 @@ export class Batch {
 			} catch (e) {
 				reset_all(root);
 				// If there's no async work left, this branch is now dead and needs
-				// to be unlinked to not become a zombie that is never cleaned up.
+				// to be discarded to not become a zombie that is never cleaned up.
 				// See https://github.com/sveltejs/svelte/issues/18221#issuecomment-4497918414
 				// for a (non-minimal) reproduction that demonstrates a case where this is necessary
 				// to not get follow-up false-positives via "batch has scheduled roots" invariant errors.
-				if (!this.#is_deferred()) this.#unlink();
+				if (!this.#is_deferred()) this.discard();
 				throw e;
 			}
 		}
@@ -852,7 +864,6 @@ export class Batch {
 	static ensure() {
 		if (current_batch === null) {
 			const batch = (current_batch = new Batch());
-			batch.#link();
 
 			if (!is_processing && !is_flushing_sync) {
 				queue_micro_task(() => {
@@ -972,18 +983,11 @@ export class Batch {
 		this.#roots.push(e);
 	}
 
-	#link() {
-		if (last_batch === null) {
-			first_batch = last_batch = this;
-		} else {
-			last_batch.#next = this;
-			this.#prev = last_batch;
-		}
-
-		last_batch = this;
-	}
-
 	#unlink() {
+		// #merge calls #unlink, discard later on does it again - prevent
+		// running it multiple times to not corrupt the linked list
+		if (!this.linked) return;
+
 		var prev = this.#prev;
 		var next = this.#next;