Add URRobotCfg: UR family (ur3/3e/5/5e/10/10e)

[yuecideng]

Description

Adds URRobotCfg — a single RobotCfg subclass covering the Universal Robots UR family (UR3, UR3e, UR5, UR5e, UR10, UR10e), selected via robot_type. Built with the /add-robot skill on top of the unified robot protocol from #330.

from embodichain.lab.sim.robots import URRobotCfg
cfg = URRobotCfg.from_dict({"robot_type": "ur5"})
robot = sim.add_robot(cfg=cfg)

Design

• One class, six variants — robot_type picks the URDF, joint-name casing, and per-variant drive defaults. The kinematic (DH) parameters are owned by URSolverCfg (its __post_init__ already sets them per ur_type); the robot config owns the URDF, control parts, drive properties, and rigid-body attributes.
• Scale-aware defaults — drive_pros.max_effort is sized per variant (UR3/UR3e ≈ 56 N·m, UR5/UR5e ≈ 150 N·m, UR10/UR10e ≈ 330 N·m; sim defaults, not factory specs).
• UR5 joint-name quirk — the UR5 URDF uses lowercase joint1–joint6 while every other variant uses Joint1–Joint6; _build_defaults selects the correct casing per robot_type.
• Follows the protocol — 3-line from_dict, variant-aware _build_defaults, build_pk_serial_chain routed through _pk_urdf_path, inherited to_dict/save_to_file (round-trips).

Prerequisite fix

URSolverCfg did not set class_type (it inherited "BaseSolver" from SolverCfg), so from_dict round-tripping looked up a nonexistent BaseSolverCfg and failed. Added class_type: str = "URSolver" (matches the OPWSolverCfg/SRSSolverCfg pattern). This was a latent bug — URSolverCfg could not be deserialized from a dict before. The existing UR solver IK tests still pass.

Docs

• New docs/source/resources/robot/ur_robot.md + registered in resources/robot/index.rst.
• __main__ smoke test in ur_robot.py (add to sim + round-trip check).

Dependencies

• Stacked on Unify robot config protocol + add-robot skill #330 (the unified RobotCfg protocol: _build_defaults hook, merge_robot_cfg variant-key filtering, inherited to_dict). This PR targets feat/robot-unified-protocol; once Unify robot config protocol + add-robot skill #330 merges, it can be retargeted to main.

Dependencies: none beyond #330.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Enhancement (non-breaking change which improves an existing functionality)
• Breaking change (existing functionality will not work without user modification)
• Documentation update

Screenshots

N/A.

Checklist

• I have run the black . command to format the code base.
• I have made corresponding changes to the documentation (new ur_robot.md + index entry).
• I have added tests that prove my feature works (tests/sim/objects/test_robot_cfg.py: 6× from_dict per variant, 6× round-trip, 6× DOF drift guard, default-type, max-effort-scales-with-size, unknown-type-raises — 21 UR tests; 28 cfg tests total green). Sim smoke: UR3/UR5/UR10 load into SimulationManager with 6-DOF arm. Existing UR solver IK tests unaffected.
• Dependencies have been updated, if applicable.

[matafela]

1.0 kg can be too heavy for links near robot flange. Should we use 0.1 kg or just use mass from urdf link inertia tag?

[yuecideng]

Actually the mass will be ignored for asset loading (we set replace_inertia=False as default). I will remove it (and the other params that can be removed as well) directly from the config,