[runtime] Route-following and trajectory follow-steering motion never landed (§6.8.2, §6.9)

[JArmandoAnaya]

What

Path-level motion never landed; the closed sprints delivered storage and routing only:

1. Route following — AssignRouteAction/AcquirePositionAction store the route but no controller follows it: core/include/scena/engine.h:468-469 "Stored, not yet followed", core/src/engine.cpp:3223-3224 "Route-following motion needs a road network (p3-s4)". p3-s4 landed the network and deterministic routing (roads/opendrive/tests/route_test.cpp), so the stated prerequisite is met and nothing owns the remainder (§6.8.2; RouteStrategy interpretation included).
2. True follow trajectory steering + closed trajectories — deferred at core/src/engine.cpp:1480-1490, core/include/scena/ir/action.h:650, core/include/scena/ir/dynamics.h:56 ("a deferred follow-up, ADR-0018") with no owner at all — neither sprint nor issue. Note [p2-s2 follow-up] Relative speed targets, follow-mode jerk & DynamicConstraints #62 delivered follow-mode for the speed actions only.

Done when

• Route-following motion implemented on the road backend (deterministic, via detmath; fixtures on the hand-authored maps incl. a junction route), or explicitly re-scoped to Post-v0.0.1 with this issue as owner
• Same decision recorded for follow-steering and closed trajectories; the coverage-matrix rows and ADR-0018 note point here instead of at nothing
• GS-8-adjacent golden coverage considered once motion exists