sim/mujoco: MjSpec stage 4 - multi-robot compose via repeated attach() (#121)

[cagataycali]

Part of #121. See IDEA.md stage 4.

Goal

Replace MJCFBuilder.compose_multi_robot_scene and _prefix_robot_names / _namespace_robot_default_classes / _collect_existing_class_names with SpecBuilder.build(world) + per-robot attach().

Current dead code

compose_multi_robot_scene is defined in mjcf_builder.py but never called from source. _prefix_robot_names is called during per-robot inject - also redundant once attach() lands.

Acceptance

• Multi-robot scenes compile identically under both flags.
• compose_multi_robot_scene, _prefix_robot_names, _namespace_robot_default_classes, _collect_existing_class_names deleted (or flagged for deletion in Stage 7).
• Integration test: 2 robots from different URDFs with different asset dirs.
• Namespace convention ({robot_name}_ prefix) verified against downstream code that reads joint/actuator names.

[cagataycali]

Delivered in PR #85.

Commits:

• ad1d298 feat(sim/mujoco): MjSpec-based builder (stages 0-2 of IDEA.md)
• c2e8826 feat(sim/mujoco): finish MjSpec refactor (stages 3-7 of IDEA.md)
• 3404809 feat(sim/mujoco): add patch_scene_mjcf structured-op MJCF mutator

Verified: hatch run test tests/simulation/mujoco/ → 428 passed, 1 skipped. Lint clean.