🎯 Changes

createQuery/createInfiniteQuery never threw errors from the render path when throwOnError was set — the throwOnError option was accepted (typed) but had no runtime effect for these hooks. This contradicts the documented behavior (docs/framework/svelte/quick-start.md): "Set throwOnError option to true to make sure errors are thrown to the <svelte:boundary> component."

Added a dedicated $effect in createBaseQuery.svelte.ts (shared by both createQuery and createInfiniteQuery) that throws when shouldThrowError returns true. The throw must happen from inside a Svelte reaction — verified experimentally that throwing from the subscribe callback (which runs through notifyManager's batching, outside any active Svelte reaction) never reaches <svelte:boundary> and instead becomes an unhandled error.

The reaction reads observer.getCurrentResult() (untracked) rather than the query result object returned to consumers, gated behind a separate resultVersion counter. Reading query.isError/query.isFetching directly would mark them tracked on the trackResult proxy the first time an error occurs, permanently widening notifyOnChangeProps for every consumer of that query from then on — verified with a regression test that reproduces the pollution (1 vs 4 re-runs) before/after this change.

This still notifies reliably because queryObserver.ts force-adds 'error' to the notified props whenever options.throwOnError is set, regardless of what any consumer has read. This is a load-bearing dependency on that query-core internal — flagging it here for visibility.

createQueries.svelte.ts is out of scope for this PR — same defect, separate change.

Also out of scope: the QueryErrorResetBoundary/errorResetBoundary reset machinery react-query has (retryOnMount suppression while unreset). Svelte 5's <svelte:boundary> gives a reset() callback to the failed snippet directly, a different primitive — mapping React's reset model onto it is a separate design decision.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested code changes locally with pnpm run test:pr, or these tests do not apply to this pull request.
• I fully understand the code in this pull request, including any code generated with AI assistance.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).