Add a `forward_euler` flag to `euler_integrate` so the shared loop can serve
the groot/evo1/wall_x policies (integrate t: 0 -> 1, dt = +1/num_steps) in
addition to the openpi backward convention (t: 1 -> 0), which remains the
default. RTC hook and debug tracking are unchanged in both directions.
Adds forward-convention equivalence tests against a reference loop.
Co-authored-by: Cursor <cursoragent@cursor.com>
Martino Russi