experimental.async: batch merging permanently swallows unrelated synchronous UI updates when an earlier async batch never settles

Describe the bug

Since 5.55.6 (#18205, "blocking" → "merging" batch strategy), a batch that shares any written (non-derived) source with an earlier still-pending batch is merged into it (#find_earlier_batch() → #merge()). Two consequences:

1. If the earlier batch never settles (a hung promise — or just a slow one, for its duration), every merged batch's DOM updates are withheld, including purely synchronous updates to unrelated parts of the UI. No error, no warning, no pending UI. The $state values are updated (reads see the new values); only the DOM commit never happens.
2. #merge() copies the merged batch's current sources into the stuck batch, so the stuck batch's "contagion set" grows with every merge. Batches that share a source only with a previously merged batch — not with anything the stuck batch originally wrote — get swallowed too. One hung async can progressively deaden the whole app, interaction by interaction.

On ≤ 5.55.5 the same interaction sequence works: the blocking strategy only blocked a batch when it intersected and differed from the earlier batch's writes, and, crucially, transitively-connected batches did not block, because #is_blocked() consulted the blocker's own blocking_pending, which is empty for a batch that merely got deferred (it has no async of its own).

Reproduction

Broken (5.56.4)

Working (5.55.5)

Logs

System Info

- svelte 5.56.4 (regression window 5.55.6 → 5.56.4; 5.55.5 OK), @sveltejs/vite-plugin-svelte 6.2.4, vite 7.3.6
- Chrome 143 / macOS (also reproduced in the svelte.dev playground)

Severity

blocking an upgrade