[shawn-maybush]

Summary

Adds interactive 3D body clicking and dragging using Viser's SceneNodeHandle.on_drag API (viser>=1.0.30), respecting the two-layer architecture between ViserMujocoScene and Viewer:

• ViserMujocoScene (src/mjviser/scene.py):
  • Introduces BodyDragEvent (exported from mjviser) and add_body_drag_callback / on_body_drag / remove_body_drag_callback / remove_body_drag_callbacks.
  • Binds drag handlers across batched dynamic mesh groups (_mesh_groups), fixed body geom handles (_fixed_geom_handles), and dynamic convex-hull handles (_hull_dynamic_handles), automatically re-binding when visual or hull handles are rebuilt.
  • Resolves batched instance_index values back to (env_idx, body_id, mocap_id) for both full-batch and show_only_selected slicing modes, and subtracts scene_offset so callback coordinates are reported in MuJoCo world space.
• Viewer (src/mjviser/viewer.py):
  • Registers default cmd/ctrl (pull & tumble) and cmd/ctrl+shift (rotate) drag callbacks backed by MuJoCo's native MjvPerturb, mjv_applyPerturbPose, and mjv_applyPerturbForce for mass-scaled, velocity-damped spring perturbations and direct mocap / paused free-body manipulation.
  • Normalizes MjvPerturb.localmass by model.vis.map.stiffness so perturbation_stiffness remains in $\text{N}/(\text{m}\cdot\text{kg})$ even when a model defines a custom <map stiffness=.../>.
  • Renders a 3D perturbation indicator line (/perturbation_force) while dragging dynamic bodies and adds a Perturbation folder under the Physics tab with Stiffness and Torque Scale sliders.
  • Clears active drag state and applied forces on simulation reset or keyframe load.

Test Plan

• uv run ruff format --check & uv run ruff check
• uv run pyright (0 errors)
• uv run pytest (83 passed, including new unit tests in tests/test_scene.py and tests/test_viewer_drag.py covering multi-env instance decoding, convex-hull handle dragging, selective callback removal, spring/torque perturbations, damping, custom <map stiffness=.../>, mocap manipulation, and paused single-stepping)

[shawn-maybush]

I closed as PR #21 is more comprehensive and further along.