fix!: waypoint rates are optional, and survive a transmission

[jimsynz]

Two fixes, found together because the first exposed the second.

Waypoint rates are optional

Command.Trajectory required velocity and acceleration on every waypoint. Positions and times is the ordinary way to describe a trajectory — it's what ROS-style planners emit — and a caller with only that had to invent rates to satisfy the schema.

Inventing is worse than omitting: a driver can't tell a made-up 0.0 from a deliberate standstill. Both fields are now optional, and absent means you decide — which a driver can act on by working out the speed needed to arrive by time_from_start.

follow_trajectory/4 also stopped multiplying a nil rate by 1.0.

Trajectories were broken on any joint with a transmission

Found while testing the above. Transmission.apply_to_waypoint/2 pattern-matched a waypoint as a map:

defp apply_to_waypoint(%{position: p, velocity: v, acceleration: a} = wp, t) do

but the schema has always declared them as keyword lists ({:list, {:keyword_list, @waypoint_schema}}). So every trajectory sent to a joint with a transmission raised FunctionClauseError and took the actuator down:

with no transmission:  ok
with a transmission:   RAISED: no function clause matching in BB.Transmission.apply_to_waypoint/2

On an SO-101 that's five joints of six. Only the gripper works, because it has no transmission — which is exactly why this survived unnoticed, and why I only caught it when I went looking for something else.

It now handles keyword lists, and leaves an omitted rate omitted rather than turning it into a zero. There was already a maybe_apply_rate/2 for precisely that; it just wasn't being used here.

Breaking

velocity and acceleration on a waypoint may now be nil. Anything reading them straight out of a waypoint needs to cope — drivers treating them as always-present want a fallback. bb_servo_feetech already has one: an absent velocity falls back to the leg's time budget, then to the joint's velocity limit.

Testing

mix check --no-retry passes; 1317 tests. Four new ones cover the keyword-list shape, an omitted rate surviving the transmission as nil rather than 0.0, time_from_start being untouched, and the nil-transmission path.

Came out of hardware work on an SO-101 — bb_servo_feetech#94 unified its position and trajectory code paths, at which point the fallback for an absent velocity turned out to be unreachable through the public API.