fix reactivity in computeds

[treardon17]

Type	Description	Icon	Changelog
fix	Fixes an issue	🐛	✅

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Fix: Dependency tracking lost when creating reactive objects during computation

Problem

When a computed/observer creates and modifies reactive objects during its execution, dependencies on the original reactive sources were being incorrectly cleared.

This occurred because:

1. Computed accesses sourceAtom.value → registers dependency under key 'value'
2. Computed creates a new atom and reads from it → registers another 'value' dependency
3. Computed writes to the new atom → triggers trackerChanged(newAtom, 'value')
4. trackerChanged called observerClear(obs, 'value') unconditionally
5. This cleared all 'value' dependencies, including the original sourceAtom

This pattern is common with reactive decorators, where each property uses an internal atom with 'value' as the tracking key.

Solution

Only call observerClear when the observer is NOT currently running. If it's mid-execution, it's still collecting dependencies and we must not clear them.

if (!isObserverRunning(obs)) {
  observerClear(obs, key);
}

Added a WeakSet to track running observers for O(1) lookup performance.

Changes

• src/reactivity/Observer.ts: Added isObserverRunning() helper and RUNNING_OBSERVERS WeakSet
• src/reactivity/global.ts: Guard observerClear call with isObserverRunning check
• src/reactivity/spec/observer.spec.ts: Added 3 regression tests for the read-then-write pattern

Test Plan

• All 399 existing tests pass
• New regression tests fail without fix, pass with fix
• Manually verified fix resolves the original issue in consuming application

📚 Bookkeeping:

Testing (if applicable):

• Ran/wrote unit tests for this

Checklist

• Assigned PR to myself
• Added at least 1 person on the team as reviewer
• Release Notes: PRs types that have the 🗒️ next to them also require release notes to be added to the CHANGELOG.md

[bitencode]

🎉 Thanks! Looks good 😄