node:process: implement child-side IPC channel surface #2529

Summary

Perry still needs the child-side process IPC surface when a compiled Perry program is launched with a Node-style IPC channel (NODE_CHANNEL_FD).

The parent-side child_process.fork() IPC work landed in #1955/#1933, but that PR explicitly called out the compiled-Perry child side as a remaining follow-up. Today the lowering path appears to treat process.send, process.disconnect, and process.connected as unconditionally undefined, which matches a normal non-IPC process but cannot support a Perry child process inheriting fd 3.

Evidence

• docs/runtime-parity.md:2275-2279 lists process.send, process.disconnect, process.channel, process.channel.ref(), and process.channel.unref() as public Node process IPC APIs.
• docs/runtime-parity-gaps.md:635-639 keeps the same process IPC rows in the gap list.
• crates/perry-hir/src/lower/expr_member.rs short-circuits process.send, process.disconnect, and process.connected to undefined for ordinary process member reads.
• crates/perry-runtime/src/child_process/fork.rs launches forked children with NODE_CHANNEL_FD=3 so Node children get process.send / process.on("message") automatically, but this does not install the same surface for a compiled Perry child.
• PR feat(node:child_process): async spawn reactor, fork()+IPC, exec error/encoding parity (#1933–#1938) #1955 says the remaining follow-up is the child-side process.send / process.on('message') behavior when a compiled-Perry binary is itself the fork target.

Expected behavior

When a Perry process starts with a valid Node-style IPC channel, it should expose Node-compatible child-side IPC APIs:

• process.send(message[, sendHandle[, options]][, callback]) serializes messages to the parent.
• process.on("message", handler) receives parent messages.
• process.disconnect() closes the IPC channel and updates state.
• process.connected reflects channel state.
• process.channel exposes the channel object, including ref() / unref().
• In a normal non-IPC process, these members should continue to match Node's non-IPC shape.

Duplicate check

I searched issues and PRs for compiled Perry child process.send NODE_CHANNEL_FD process.on message and process.send NODE_CHANNEL_FD process.channel compiled. The only related result is merged PR #1955 / closed #1933 for parent-side child_process.fork() IPC, and that PR explicitly leaves this compiled-child-side behavior as follow-up work.

Verification note

I do not have a local perry binary in this checkout, so this is based on committed parity docs/tests, source search, and the #1955 PR notes rather than a fresh runtime comparison.