Formalize manipulation time-parameterization stage in PlanningSpec

Context

Manipulation planning currently treats GeneratedPlan as selected global joint waypoints, while execution later converts that timeless path into per-robot JointTrajectory objects through JointTrajectoryGenerator.generate(...) inside ManipulationModule.execute_plan().

Actual issue

Time-parameterization is not formalized as an explicit planning pipeline stage. The boundary between geometric planning and time-parameterized trajectory generation is implicit and currently lives in module execution/preview code. This makes preview timing, execution timing, and planner outputs easy to drift apart.

Proposed direction

• Add an explicit time-parameterization stage to the manipulation planning pipeline.
• Make that stage part of PlanningSpec / planning stack responsibility, not ad hoc ManipulationModule responsibility.
• Define the data model boundary clearly:
  • geometric selected-joint path / GeneratedPlan before time-parameterization
  • time-parameterized trajectory artifact after time-parameterization
• Ensure preview and execution consume the same time-parameterized artifact or the same formal time-parameterization service.
• Preserve planner-provided timestamps when available, but handle untimed geometric planners like current RRT through the formal time-parameterization stage.

Acceptance criteria

• Planning stack exposes an explicit time-parameterization responsibility/API through PlanningSpec or an adjacent planning-stage spec.
• ManipulationModule no longer owns ad hoc path-to-trajectory timing logic.
• Preview and execution use the same formal time-parameterized representation/path.
• Data model and tests distinguish geometric paths from time-parameterized trajectories.
• Untimed planner outputs and planner-provided timestamps are both covered by tests.

Synced from DIM-1027 by che

Trace PlanningSpec, GeneratedPlan, JointTrajectoryGenerator.generate(...), and ManipulationModule.execute_plan() to map where geometric paths become trajectories. Define the boundary between untimed paths and time-parameterized artifacts, then verify that preview and execution share it. Done means the planning stack exposes this responsibility and tests cover both untimed planner output and preserved planner timestamps.