Fix pipeline velocity corrupting global speeds and mid-track path origin

[cursor]

Bug and impact

1. Global reference speeds permanently zeroed — With LocalPlanningPipeline + VelocityLocalPlanner and an empty path stage, plan_velocity profiled the shared global TrajectoryTracker in place. After braking for an obstacle, clearing the obstacle left the global velocities at 0, so the ego never resumed cruise.

2. Mid-track local path started at the origin — ReferencePathPlanner.plan_path copied the full route from s=0 with trajectory=None, so the velocity stage / controllers rebuilt a tracker at current_wp=0 while ego was mid-route (e.g. after a corner). Local plans aimed at the wrong segment and emergency-braked.

Root cause

• LocalPlan.from_trajectory aliases the live global tracker; velocity profiling mutates it. ReferencePathPlanner avoided that alias but still emitted geometry from route start.
• Standalone VelocityLocalPlanner.replan already cloned via slice_trajectory_horizon; the pipeline velocity stage did not.

Fix

• VelocityLocalPlanner.plan_velocity profiles a copy (path/velocity/waypoints/boundaries).
• ReferencePathPlanner.plan_path emits the forward horizon from current_wp (slice_trajectory_horizon(..., max_points=0)).

Validation

PYTHONPATH=/workspace python3 -m pytest \
  test/c20_planning/test_c23_local_planning_pipeline.py \
  test/c20_planning/test_c27_local_behavioral_and_velocity_planners.py -v

31 passed (includes new regressions for global-speed preservation and mid-track reference path).

Not re-reported (still open)

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• PR Fix corridor-only collision index delaying braking to path end #30 — corridor-only collision index
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