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.

This PR was opened by the [Changesets
release](https://github.com/changesets/action) GitHub action. When
you're ready to do a release, you can merge this and the packages will
be published to npm automatically. If you're not ready to do a release
yet, that's fine, whenever you add more changesets to main, this PR will
be updated.


# Releases
## svelte@5.55.10

### Patch Changes

- fix: unlink errored and otherwise finished batch
([#18264](#18264))

- perf: walk composedPath() directly in delegated event propagation
([#18268](#18268))

- fix: transfer effects when merging batches
([#18254](#18254))

- fix: allow `$derived(await ...)` in disconnected effect roots
([#18273](#18273))

- fix: remove temporary raw-text hydration markers
([#18269](#18269))

- fix: propagate async `@const` blockers through closure references so
template expressions like `{(() => host)()}` correctly wait for the
awaited value ([#18309](#18309))

- fix: properly unlink batches
([#18298](#18298))

- fix: settle discarded batch
([#18290](#18290))

- fix: declare `let:` directives before `{@const}` declarations on
slotted elements
([#18271](#18271))

- fix: resume outro-ed branches if they were kept around
([#18291](#18291))

- fix: avoid waterfall-warning when async resolves to same value
([#18297](#18297))

- fix: correctly coordinate component-level effects inside async blocks
([#18260](#18260))

- fix: make unnecessary commit work less likely
([#18263](#18263))

- chore: add tag name to `a11y_click_events_have_key_events` warning
([#18272](#18272))

- fix: catch rejected promises while merging/committing
([#18266](#18266))

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