One-shot helper that opens a PTY, forks a child, execvp's the given
argv with the slave as stdin/stdout/stderr, and hands the master fd
back to the caller over a unix socket via SCM_RIGHTS. Wire contract
is stdin JSON → stdout JSON + fd transfer per D-005; socket fd
defaults to 3 with PTYC_SOCK_FD override for language runtimes whose
subprocess machinery shuffles the low fd numbers.
Minimal JSON parser (no deps) scoped to the exact accepted shape.
Exec-failure pipe (CLOEXEC) reports child-side errors back without
zombies. Window size applied via TIOCSWINSZ before fork; child
becomes session leader + makes the slave its controlling TTY.
Root Makefile PTYX_PRESENT typo fixed → PTYC_PRESENT, and a
ptyc-test target added alongside ptyc-build / ptyc-clean. Eight
smoke tests pass (happy path, env replacement, cwd, window size,
bad argv, type errors, exec failure, unknown keys).
Co-Authored-By: Claude <noreply@anthropic.com>
