fix: re-subscribe to ordinary external source after a transition

[SnowingFox]

Problem

A computation created while a startTransition is pending never receives more than one external update when enableExternalSource is enabled. In an app this shows up as a subtree that renders correctly, updates once, then stays stale until something else forces it to re-run.

Root cause

When ExternalSourceConfig is set, createComputation (in packages/solid/src/reactive/signal.ts) wraps the computation's fn so that a run while a transition is pending tracks the computation on a transition-scoped external source (inTransition) instead of the ordinary one. A computation created during a transition only ever has inTransition.track() called, so its ordinary source records zero dependencies. After the transition completes, triggerInTransition's .then() disposes the transition-scoped source and nothing ever re-registers the computation on the ordinary source — the external system can no longer invalidate it.

Fix

Track whether the computation has ever run through the ordinary external source. When the transition-scoped source is disposed, if it never did, fire the computation's trigger once so it re-runs through the ordinary source and re-subscribes. If the computation was disposed in the meantime the trigger is a no-op, because the computation is no longer an observer of the internal track signal.

Test

Adds a regression test to packages/solid/test/external-source.spec.ts that creates a memo inside a transition, applies two external updates, and asserts both are delivered. On main the second update is lost and the test fails.

Fixes #2953

[changeset-bot]

🦋 Changeset detected

Latest commit: 619e252

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

This PR includes changesets to release 2 packages

Name	Type
solid-js	Patch
test-integration	Patch

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