node:child_process: honor subprocess.send callback and closed-channel semantics #3316

Summary

fork() now exposes a parent-side subprocess.send(...), but Perry does not match Node's observable optional-argument and closed-channel behavior. In Node, send has the message[, sendHandle[, options]][, callback] shape, invokes the callback when delivery succeeds or fails, and reports closed IPC channels. Perry currently ignores the optional argument slot and returns success regardless of whether the IPC write succeeded.

Node Behavior

Local Node v25.9.0 probe:

const { fork } = require("node:child_process");
const child = fork(childFile, [], { stdio: ["ignore", "ignore", "ignore", "ipc"] });

console.log(child.send.length); // 4
console.log(child.send({ ok: true }, (err) => console.log(err && err.code))); // true, callback gets null
child.disconnect();
console.log(child.send({ after: true }, (err) => console.log(err && err.code))); // false, callback gets ERR_IPC_CHANNEL_CLOSED

The same API also accepts the documented sendHandle and options slots for IPC handle passing / serialization options.

Perry Behavior

Current source evidence:

• crates/perry-runtime/src/child_process/fork.rs:95 installs send on forked ChildProcess objects as cp_cast2(cp_method_send).
• crates/perry-runtime/src/child_process/mod.rs:873-882 implements cp_method_send(closure, message, _arg), ignores _arg, never detects a callback/options/sendHandle shape, calls reactor::cp_ipc_send(handle, message), discards that boolean result, and always returns true.
• crates/perry-runtime/src/child_process/mod.rs:985 registers the method arity as 2, while Node reports child.send.length === 4.
• crates/perry-runtime/src/child_process/reactor.rs:328-378 can report IPC write failure as false, but the public send method does not surface it or deliver a callback error.
• Existing closed node:child_process — fork() + IPC (send/disconnect/'message'/channel) unimplemented #1933 covered making parent-side fork IPC exist; it did not track this callback / closed-channel / optional-argument behavior as a remaining leaf.

Suggested PR Cut

Batch this with related issues in:
node:child_process: subprocess lifecycle and setup parity cut

Good batch candidates:

• node:child_process: 57 surface methods still stubbed (ChildProcess.kill/disconnect/send + stdio + execFile) #2130
• node:child_process: implement subprocess[Symbol.dispose] lifecycle method #2556
• node:process: implement child-side IPC channel surface #2529

Do not batch with:

• unrelated modules
• broad subsystem rewrites
• spawn/exec option semantics that do not touch IPC delivery
• fixes needing a compiled-Perry child-process IPC harness unless the PR deliberately includes that harness work

Acceptance

• parity/regression test proves callback invocation on successful subprocess.send(...)
• parity/regression test proves a disconnected IPC channel returns false and calls the callback with a Node-shaped ERR_IPC_CHANNEL_CLOSED
• subprocess.send.length matches Node's observable arity
• implementation handles or explicitly rejects unsupported sendHandle / options cases with Node-shaped behavior instead of silently ignoring them
• related known-failure/docs/manifest entries updated if touched