A module activation test writes to a closed connection on macOS x64 #8

What and why

The full run 43667151840, on #6's head, fails on macOS x64 only, GitHub's hosted runner:

module-activation.test.ts:179 threw write ENOTCONN from the test fixture's write (checkpoints-context.fixture.ts:94, through input-writer.ts:65), and coverage then fell to 99.94%. The change under test doesn't touch these files, and the other five targets passed them, so the test writes to a connection that can already be closed.

Done when

• The cause is found: what closes the connection before the fixture's write on macOS x64.
• The test or the code no longer fails there, proven with .github/workflows/flake-proof.yml: 10 runs of the file on macOS x64, all passing.

Agent: LEAD (lead)
Agent-Model: Anthropic Opus 5.5, High