v3.5.0

Timed trajectories for execution (#214). Given joint velocity and
acceleration limits, sscbirrt now also returns a trajectory along the planned
path. It carries velocity through corners and cuts them where the planning
problem allows. On the pick demo the median duration falls from 6.23 s (a
stop at every corner) to 3.61 s. PR #211's blends retimed by TOPP-RA reach
4.47 s, and they fail validation on 8% of seeds.

Added

• trajectory=TrajectoryOptions(...) on CBiRRT.solve, CBiRRT.plan
  (with return_details=True) and sscbirrt.mujoco.plan sets
  result.trajectory. CBiRRT.smooth_trajectory(problem, path, options)
  times a path you already have. result.path is unchanged.

• sscbirrt.Trajectory: the exact source law. It provides:

  • evaluate, which gives q, qdot and qddot;
  • the boundary states start and goal;
  • the limits used, joint_names and the validation resolution;
  • seed, backend and why it stopped;
  • a validation record on every piece ("planned" or "shortcut").

  sample(dt_ns) gives a runtime command. It is a discretization, which the
  consumer validates under its own interpolation.

• sscbirrt.TrajectoryOptions: the limits (required; a scalar or one per
  joint) and named stopping defaults:

  • stop once the last 40 attempts shortened the trajectory by less than 0.5%;
  • at most 400 attempts;
  • no time budget.

  The defaults come from a stopping-rule study
  (tools/trajectory_benchmark.py trace / stopping). Every rule except
  time_budget depends only on the trajectory, so a seed gives the same
  trajectory on any machine. abort_fn also stops smoothing.

• Native trajectory smoother (sscbirrt::trajectory in the C++ core).
  It mirrors the Python reference operation by operation, and only the
  random stream differs. Parity tests hold the ramps, synchronization and
  corner merging to 1e-12. backend="auto" uses it when the problem lowers.

• tools/trajectory_benchmark.py: #214's adoption benchmark and gates, and
  offline replay of stopping rules. tools/blend_baseline.py is a frozen copy
  of PR #211's blends, for the TOPP-RA baseline only. Results are in
  benchmarks/trajectory/.