feat(kernel): added PTY subsystem, proc_set_handle API, and reusable line discipline

[FlareCoding]

Note

Medium Risk
Touches kernel resource/handle management and adds new syscalls and terminal I/O paths; bugs could cause handle leaks, incorrect rights propagation, or PTY/console read-write/close edge cases.

Overview
Adds a new PTY resource type and subsystem, including pty_create syscall to allocate a master/slave pair backed by ring buffers and a per-PTY line discipline (with cooked/raw mode and optional echo).

Extends process management with proc_set_handle, enabling callers to replace a specific handle slot in a CREATED child before proc_start, supported by new handle-table helpers (get_handle_object can return rights; install_handle_at replaces an existing slot safely).

Refactors console input handling into a reusable terminal/line_discipline module and updates the console terminal to use it; adds kernel PTY unit tests and a userland ptytest app plus libstlx wrappers and syscall number definitions.

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