node:child_process: implement subprocess[Symbol.dispose] lifecycle method #2556

Summary

Current Node exposes subprocess[Symbol.dispose]() on ChildProcess; as of Node v24.2.0 it is no longer experimental and it calls subprocess.kill('SIGTERM'). Perry's child-process objects expose kill, ref, unref, and IPC methods, but do not expose or dispatch the well-known disposer method for child processes.

This leaves using child = spawn(...) / explicit child[Symbol.dispose]() cleanup paths incompatible even though the adjacent timer disposer support already exists.

Evidence

• Current Node docs document subprocess[Symbol.dispose]() as stable and define it as calling subprocess.kill() with SIGTERM: https://nodejs.org/api/child_process.html#subprocesssymboldispose
• test-files/test_parity_child_process.ts:125 explicitly skips subprocess[Symbol.dispose]() because symbol-key dispatch for this surface is not yet uniform.
• Live spawn() ChildProcess objects are constructed with methods on, once, addListener, prependListener, removeListener, off, removeAllListeners, emit, kill, ref, and unref, but no disposer: crates/perry-runtime/src/child_process/reactor.rs:369-384.
• fork() ChildProcess objects add send and disconnect, but likewise omit a disposer: crates/perry-runtime/src/child_process/fork.rs:72-89.
• The generic native method dispatch has special well-known-dispose handling for timer handles only (__perry_dispose__ / @@__perry_wk_dispose clear timeout/interval/immediate IDs): crates/perry-runtime/src/object/native_call_method.rs:344-395.

Duplicate check

Searched issues and PRs for:

• subprocess Symbol.dispose child_process dispose child.kill SIGTERM
• child_process Symbol.dispose
• subprocess dispose

No open or closed tracker for this child-process disposer gap was found. This is separate from #2555's option-semantics issue and from closed spawn / fork IPC work (#1933/#1934).

Acceptance sketch

• child[Symbol.dispose]() exists on ChildProcess objects from both spawn() and fork().
• Calling it invokes the same path as child.kill('SIGTERM') and returns undefined.
• The well-known symbol lowering path (@@__perry_wk_dispose / class-member equivalent) routes to child-process disposal without affecting existing timer disposal behavior.
• Add a deterministic parity test that creates a long-lived child, calls the disposer, and observes termination/cleanup without hanging.

Note: this issue was confirmed from committed source and current Node docs; no local target/release/perry binary was available in this checkout for a fresh runtime probe.