fix(interpreter): run coprocesses concurrently over in-process pipes

[chaliy]

Requested by Михайло · project thread

What changed

• coproc runs its body as a background job alongside the shell, connected by two bounded in-process pipes (the same pipes pipeline stages use). Writes to ${NAME[1]} feed its stdin, closing the write end delivers EOF, reads from ${NAME[0]} get its output.
• Request/response loops work without deadlock; read -u, wait $NAME_PID (returns the body's status), $!, jobs, the default name COPROC, bash's fd numbers (63 60) and the "still exists" warning match bash.
• After wait reaps a coproc, NAME/NAME_PID are unset and its fds closed. exec() closes unclosed coprocs so a call can't hang.
• <&$x with an empty $x is now an error instead of silently reading stdin.

Why

Level-up roadmap, concurrency/streaming step: closes the coproc-basic bash-oracle gap.

Before / After (verified)

bash-oracle: match 102 → 103, strict 100 → 101 / 106. floor.txt raised to 103 101.

coproc-basic (coproc UPPER { tr a-z A-Z; }, write, close, read): main printed an empty line, bash and this branch print TEXTO.

coproc C { while read l; do echo "got $l"; done; }
for i in 1 2; do echo "q$i" >&"${C[1]}"; read -r r <&"${C[0]}"; echo "$r"; done
exec {C[1]}>&-
wait $C_PID; echo rc=$?

bash 5.2 and this branch: got q1 / got q2 / rc=0.

Risk

• Each coproc takes a background-job slot (TM-DOS-122) and two persistent fds (TM-DOS-063); pipes are bounded (4 KiB plus one command's output). With concurrent_jobs(false) the body runs to completion at coproc with stdin at EOF.
• No new awaits in hot async fns; stack for depth_32_no_stack_overflow is identical to main.
• Remaining gaps in L-PROC-005: a finished coproc stays until wait (bash reaps on SIGCHLD); a coproc fd as a compound command's stdin is read up front; read -n on a coproc fd consumes the whole line; writes after exit are dropped instead of SIGPIPE.

Checklist

• Failing tests first (coproc_tests.rs), verified against bash 5.2
• fmt, clippy, integration, lib, terminal, CLI, bash_comparison, rustdoc, check-okf
• knowledge updated (parallel-execution, limitations, threat-model, log)

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