fix: end a restored reaction context at the end of its synchronous segment

[Nic-Polumeyv]

When an async expression resumes after a pickled await, the thunk returned by save() in reactivity/async.js calls restore() to re-arm active_reaction for the rest of the expression, then disarms it with queue_micro_task(unset_context). Any microtask already queued before that one runs inside the restored context. If it writes to a source, set() throws state_unsafe_mutation in production, since the guard is not dev-only. #18453 introduced the queued disarm and noted this case in review as unavoidable. SvelteKit hits it in practice: its fetch continuations write to internal $state (sveltejs/kit#16914), and a user's $derived((await q()).length) resuming in the same tick makes that write throw and drops the update signal.

The context restored by a save thunk now ends with the synchronous segment it was restored in. A restored flag is set by the thunk and consumed on entry to save and track_reactivity_loss, so every suspension ends it; once an expression contains a pickled await, the analysis pickles every later await in it too (has_pickled_await on ExpressionMetadata), so a trailing await compiles to $.save rather than a bare await. At the end of the body, async_thunk in 3-transform/client/utils.js wraps the return expression in $.unsave(...) when the metadata has a pickled await. If the body throws instead, the context is unset by async_derived's existing finally, as before. The queued microtask in save is removed.

Output is unchanged for expressions that pickle nothing ($derived(await a) compiles byte for byte the same). Expressions with a pickled await gain one $.unsave( call per body, and their trailing await becomes a $.save, 4 to 6 bytes gzipped in the added tests. At runtime a boolean write replaces a queued microtask per resume. bench:compare shows no difference outside run-to-run noise.

Two runtime tests reproduce the throw without any library involved, one in dev and one with the prod await shape, and fail on main.

[pkg-svelte-dev]

Install the latest version of svelte from 23298d5:

pnpm add https://pkg.svelte.dev/svelte/c/23298d5e711d9ddf18dfcec509b7e3709322863b

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/svelte/pr/18694

[changeset-bot]

🦋 Changeset detected

Latest commit: 23298d5

The changes in this PR will be included in the next version bump.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-18694-svelte.vercel.app/