When ers exits unexpectedly the user is left with no borders and no
indication anything went wrong, since BorderManager just stores the
child handle and never reaps it. Add a health_check called from the
50ms poll-timer tick (throttled to ~1Hz) that try_waits on the child,
respawns on death, and rate-limits to 5 restarts per minute followed
by a 5-minute backoff so a permanently-broken ers doesn't fork-bomb.

When a directional focus jumps diagonally (e.g. Up from a tile with no
window directly above it), the bidirectional return memory must not
record the inverse on the opposite axis — otherwise pressing the
opposite direction later routes to the perpendicular tile instead of
the natural axis-aligned target. Gate the inverse write on
perpendicular-axis overlap between source and target.