refactor: Split process child module (#13014)
## Why
The process child module mixed public process management,
platform-specific handle internals, IO, shutdown semantics, and tests in
one large file. Splitting those responsibilities reduces context needed
to review future process changes.
## What
Moves child handle internals, IO helpers, shutdown types and processing,
state manager/channel logic, PTY test guard, and tests into sibling
modules under `crates/turborepo-process/src/child/` while preserving the
public API from `child.rs`.
## How
This is intended as a mechanical refactor. Verified with `cargo fmt -p
turborepo-process`, targeted child tests, `cargo test -p
turborepo-process -- --test-threads=1`, `git diff --check`, and the
repository pre-push hook.
