tests: test-invocation-lifetime reads PID files before they are written, so a stopped-compiler check can pass on PID 0 #3432

Context

Found while fixing #3401. That issue's storage-inventory tests waited for a child's PID file with existsSync, then parsed it. writeFileSync creates the file before writing it, so a descheduled child could expose an empty file, and Number("") is 0. The same pattern exists in tests/test-invocation-lifetime.test.ts:

• The fake tsc publishes tmp/compiler-pid with writeFileSync (line 47). The test waits on existsSync and reads it (tests/test-invocation-lifetime.test.ts:51-52). It then asserts OWNED_COMPILER_STOPPED with !existsSync(\/proc/${pid}`)(line 56). Withpid === 0, /proc/0` never exists, so the assertion passes vacuously and the test no longer checks that the owned compiler stopped.
• tmp/survivor-pid is published with writeFileSync (line 68), awaited with existsSync and parsed (tests/test-invocation-lifetime.test.ts:71-72).

This is a false-green risk rather than a flake: the check can silently prove nothing under load. Not observed failing in CI. Found by auditing the bug class.

Acceptance criteria

• Every PID file in tests/test-invocation-lifetime.test.ts that the test polls for and then parses is published atomically (temporary file then rename), or the reader accepts only a complete, newline-terminated positive PID (the shape tests/workspace-lease.test.ts already uses).
• A forcing receipt shows the base reading PID 0 (a widened window between create and write), and shows the fix rejecting or never seeing the empty file.
• A mutation receipt shows OWNED_COMPILER_STOPPED still fails when the owned compiler is left running.

Refs #3401.