feat(signals): raised SIGPIPE for writes with no reader

[FlareCoding]

Summary

• Writes to a closed pipe or shut-down TCP stream only reported EPIPE, so pipeline writers that never check errors looped forever instead of dying the POSIX way.
• The two failing write sites now raise SIGPIPE in the writer, with the existing disposition machinery handling the rest: default kills with a signaled wait status, SIG_IGN drops it so EPIPE surfaces, and an installed handler is delivered with the EPIPE result.
• Pty writes stay signal-free (Linux gives ptys EIO-class errors) and MSG_NOSIGNAL is deferred until a TCP sendto op exists, since write() has no flag path on Linux either.

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