sim/mujoco: MjSpec stage 5 - port scene_ops inject_/eject_ to spec.recompile() (#121)

[cagataycali]

Part of #121. See IDEA.md stage 5.

Goal

scene_ops.inject_object_into_scene today:

# tmpdir, save XML via mj_saveLastXML, ET.parse, find worldbody,
# append child, delete <keyframe>, write, reload from path

Target:

def inject_object_into_scene(world, obj):
    spec = world._backend_state['spec']
    SpecBuilder._add_object(spec, obj)
    world._model, world._data = spec.recompile(world._model, world._data)
    return True

Also port eject_body_from_scene, inject_camera_into_scene, eject_robot_from_scene.

Known gotcha

recompile(model, data) preserves qpos only when joint dims are unchanged. Adding a freejoint changes nqpos (new 7 slots). Current code deletes <keyframe> elements. MjSpec has spec.keys - need to either clear or resize.

Acceptance

• All 4 inject_*/eject_* helpers have a spec-path alongside the legacy ET-based path (flag-gated).
• Policy state (qpos) preserved across a mutation when no joint dims change.
• Keyframe-dim mismatch from adding a freejoint handled cleanly (no 'qpos size mismatch' errors).
• _patch_xml_paths, _rewrite_mesh_paths, _get_abs_meshdir, _save_and_patch_xml deleted once all callers migrated (or marked for Stage 7).

[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.