feat(pty): per-channel window size

[FlareCoding]

Summary

• TIOCGWINSZ hardcoded 80x24 and TIOCSWINSZ was a silent no-op, so terminal emulators could not report their real geometry to pty clients.
• Each pty channel now stores a winsize, defaulting to the same 80x24 so behavior is unchanged until a terminal sets it.
• The termios ioctl handler is endpoint-agnostic now and wired into the master ops table too, since termios and winsize are channel-level state and emulators hold the master end.

Supersedes #141, which became unmergeable after GitHub flagged it as a stack.

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