fix(signals): defer onSettled fallback cleanup to owner disposal (closes #2766)

[tsushanth]

Summary

Closes #2766.

onSettled falls back to the global queue when the current owner is null or has CONFIG_CHILDREN_FORBIDDEN. The most common trigger for that fallback is onSettled() called from inside another onSettled() callback (the outer callback runs as a tracked effect, which sets CONFIG_CHILDREN_FORBIDDEN). The previous fallback shape:

globalQueue.enqueue(EFFECT_USER, () => {
  const cleanup = callback();
  …
  cleanup?.();        // ❌ runs the cleanup eagerly in the same flush
});

invoked the returned cleanup eagerly inside the same flush, immediately tearing down helpers whose intent was "install on settle, tear down on dispose." Composable helpers using onSettled internally — subscription setup, listener install, focus management — set up and ripped themselves down in the same tick.

Capture the calling owner and, when the callback returns a function, register it via cleanup() under that owner with runWithOwner. Cleanup now fires when the captured owner is disposed, matching the contract of the non-forbidden path. Orphaned helpers (no owner at all) drop the cleanup rather than run it immediately — there is no point at which firing it would be correct.

Test plan

Added defers cleanup from nested onSettled to owner disposal (#2766) to tests/onSettled.test.ts mirroring the reporter's scenario:

function useSubscription() {
  onSettled(() => {
    log.push("subscribed");
    return () => log.push("unsubscribed");
  });
}
const dispose = createRoot(stop => {
  onSettled(() => { useSubscription(); });
  return stop;
});
flush();
expect(log).toEqual(["subscribed"]);
dispose();
expect(log).toEqual(["subscribed", "unsubscribed"]);

pnpm --filter @solidjs/signals build
cd packages/solid-signals && npx vitest run tests/onSettled.test.ts

Result: 13/13 pass with the fix; reverting signals.ts to upstream and re-running shows the new case fails (expected ["subscribed","unsubscribed"] to equal ["subscribed"]) — confirming the test isolates the regression.

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