fs/devices — PTY subsystem, separate stdio nodes, nonblocking stdin

[wdcui]

Summary

Full device filesystem overhaul adding pseudo-terminal (PTY) support and improving existing device handling.

Stacked PR 7 of N — targets wdcui/stacked/pr6-mm-address-space (PR #746).

Key changes

• PTY subsystem: PtyManager, PtyPair shared state with master↔slave ring buffers, line discipline (ICRNL, ECHO, ONLCR), reference-counted open FDs with HUP notifications
• Separate stdio NodeInfo: stdin/stdout/stderr now have distinct inode numbers (9/10/11) instead of sharing one
• /dev/tty: Controlling terminal device that reads from stdin, writes to stdout
• Nonblocking stdin: Reads respect O_NONBLOCK via read_from_stdin_nonblocking
• Pollable wrappers: StdinPollable, PtyMasterPollable, PtySlavePollable for epoll/poll/select
• DeviceStatusFlags: Per-entry status flag tracking (O_NONBLOCK etc.)
• Manual FdEnabledSubsystemEntry: Custom on_dup/on_close for PTY refcount management across dup/fork
• *_at stubs: All fd-relative methods return NotADirectory (devices aren't directories)
• Directory stat: /dev and /dev/pts paths return directory-style FileStatus
• Error handling: Proper error returns in open() and read() instead of unimplemented!()

Design decisions

• PTY-specific methods (get_pty_termios, set_pty_termios, get_pty_foreground_pgrp, set_pty_foreground_pgrp) are inherent methods on devices::FileSystem, not on the FileSystem trait — they're too device-specific for the generic trait interface
• PTY trait-level interface (for layered.rs delegation) is deferred to PR 8 where the first generic caller is introduced

Stats

1 file changed, 872 insertions(+), 86 deletions(-)