fix: ensure mutable atoms ignore internal no-arg updates

[davidkpiano]

🎯 Changes

Fix a regression where mutable atoms could be updated internally with no updater and have their snapshot replaced with undefined.

Mutable atoms now ignore internal no-argument _update() calls, while computed atoms keep existing recomputation behavior. This prevents external-store state from disappearing during reactive graph cleanup.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm test:pr.

🚀 Release Impact

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

[nx-cloud]

View your CI Pipeline Execution ↗ for commit 743d06d

Command	Status	Duration	Result
nx affected --targets=test:sherif,test:knip,tes...	✅ Succeeded	43s	View ↗
nx run-many --target=build --exclude=examples/**	✅ Succeeded	17s	View ↗

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☁️ Nx Cloud last updated this comment at 2026-03-06 21:05:44 UTC

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commit: fe118fa