PTY-backed terminal buffer entry held in the host’s
buffer registry. Owns the writer handle, the published
snapshot cell, and the reader task’s AbortHandle so
dropping the buffer kills the reader (and, transitively,
the child via PTY close on SIGHUP).
T3.b.2 (2026-05-25, extended T3.b.2.b): Visual-mode
selection over a terminal’s cell grid. Lines are alacritty
grid coords (negative = scrollback history; positive = live
screen); cols are cell columns. Stored on
TerminalBuffer::visual while the user is in
Terminal-Visual mode; None otherwise.
T3.b.2 (2026-05-25): Visual-mode flavour on a terminal
buffer. Mirrors lattice_grammar::VisualKind without taking
a dep on the grammar crate from the substrate.