fix: investigate intermittent pipe descriptor survivors after child close #319

After #315, the spawn failures and killed children release every pipe assertion in api/src/pipe-process.test.ts:86 intermittently fails. One API unit run found one pipe inode; one ARM64 native supervisor run found three. The assertion runs immediately after child close and all stream destroyed flags are true.

The same head passed upstream API and both native architecture jobs. I also ran the focused suite 30 times in the Linux Docker image and 300 times under the installed supervisor policy without reproducing it. This does not establish that the failures are harmless: please investigate whether Bun FileSink or readable-stream duplicate descriptors are released after the visible close event, and distinguish delayed cleanup from a persistent leak. Preserve the descriptor assertion rather than skipping it.

The missing-executable diagnostic assertion is tracked separately in #317 / #318.