[Windows port PR3] refactor: replace RawFd/AsyncFile, introduce platform-split PTY backend (#4722)

* refactor: replace RawFd with i32 in debug_to_file

Replace the Unix-specific RawFd type with plain i32 in the debug_to_file
infrastructure. This removes the last RawFd usage from the cross-crate
debug logging API, making it platform-independent.

* refactor: cross-platform PTY abstraction with AsyncFd

Replace portable-pty with native openpty and introduce a platform-split
architecture for PTY operations:

- os_input_output_unix.rs: UnixPtyBackend with native openpty, AsyncFd-based
  epoll reader (no spawn_blocking per pane), signal handling via nix
- os_input_output_windows.rs: WindowsPtyBackend stubs (unimplemented)
- os_input_output.rs: shared ServerOsApi trait, ServerOsInputOutput composes
  PtyBackendImpl, build_command() extracts TerminalAction→RunCommand logic

Key changes:
- AsyncFd<File> replaces both tokio::fs::File (upstream) and spawn_blocking
  SyncReadAsyncReader (previous port) for zero-overhead epoll PTY reads
- orig_termios restored: openpty(None, &orig_termios) passes parent's VERASE
- ServerOsApi returns Box<dyn AsyncReader> instead of Box<dyn Read + Send>
- TerminalBytes::new takes Box<dyn AsyncReader> directly
- NullAsyncReader replaces std::io::empty() for held panes
- Platform modules gated with #[cfg(not(windows))] / #[cfg(windows)]
- Removed portable-pty dependency, restored close_fds
- Added tokio "net" feature for AsyncFd

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Co-authored-by: divens <divens.dev@gmail.com>

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