fix: Eliminate flaky GC test race condition

[rekmarks]

Summary

Fixes the intermittent failures in the "should trigger GC syscalls through bringOutYourDead" test by adding proper synchronization.

Changes

• Added await waitUntilQuiescent() after queueMessage to ensure all async syscall side-effects complete before checking reference counts

Root Cause

The test was flaking due to a race condition where reference count updates from resolvePromises syscalls were still pending in microtasks when the test immediately checked them after await kernel.queueMessage() returned.

Verification

• Ran the test 10 times with --repeat 10 - all iterations passed
• Change is consistent with other synchronization patterns in the test file (lines 91, 104, 129)

🤖 Generated with Claude Code

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Note

Synchronizes the GC test to eliminate a race when asserting reference counts.

• In garbage-collection.test.ts, add await waitUntilQuiescent() immediately after kernel.queueMessage('createObject', ...) in the "should trigger GC syscalls through bringOutYourDead" test
• Ensures async syscall side-effects settle before reading kernelStore refcounts, reducing test flakiness

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