[GHSA] GHSA-5p39-cfhj-2xmp (medium) — AnyIO process-pool workers can block indefinitely on undrained stderr

GHSA-5p39-cfhj-2xmp Severity: medium CVE: CVE-2026-64847

AnyIO process-pool workers can block indefinitely on undrained stderr

### Impact AnyIO starts process-pool workers with stderr connected to a pipe but never drains that pipe. The worker redirects stdin and stdout to /dev/null to protect its protocol, but does not redirect stderr even though the documentation says all three standard streams