librio: expose cursor shape and child pid over the C ABI

Hosts that paint the cursor themselves had the position but not the
shape: DECSCUSR (block / underline / beam) and DECTCEM hidden lived only
in the Rust RenderState. rio_render_state_cursor_shape returns them as
RIO_CURSOR_*, folding "scrolled into history" into HIDDEN the way
cursor_visible() already does.

rio_surface_child_pid hands out the spawned program's pid. It is a
session leader, so a host can killpg() the whole tree on teardown;
dropping the surface only hangs up the pty and SIGHUPs that one pid,
which left editors the shell had started running as orphans.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>