Register start-bb signal handlers before spawning

[ymichael]

What was wrong

runBuildProcess spawned its build process before registering SIGINT and SIGTERM handlers. A fast child could become externally observable during that window, allowing another process to send SIGTERM while Node still had its default signal behavior. The parent then exited with signal=SIGTERM instead of cleaning up the build leader and grandchild, which caused the flaky packages job on PR #2050.

What changed

Register the build signal handlers before spawning the child. Keep spawn inside the existing try/finally lifecycle so synchronous startup failures also remove both handlers. There are no wire, daemon protocol, CLI, persistence, or documentation contract changes.

How you verified

• The first CI attempt on PR Make machine settings text-first and responsive #2050 failed in test/start-bb.test.mjs with code=null and signal=SIGTERM; its rerun passed, confirming the timing-dependent behavior.
• Repeated the focused @bb/scripts process-group integration test 100 times with Turbo cache bypass; all 100 passed without Vitest retries.
• pnpm exec turbo run test --filter=@bb/scripts --force — 18 files and 97 tests passed.
• pnpm exec turbo run typecheck --filter=@bb/scripts — passed.
• git diff --check — passed.

Follow-up to #2050.

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