Add InteractiveScene camera debug helpers (S5)

[KraHsu]

Summary

Phase S5 of the Genesis 1.0 migration (ADR-0001). Ships the camera
debug helpers — the slot in the S5 scope with a clean v1.0 implementation —
and explicitly carves the other two sub-tasks (full xacro support on
ArticulationCfg, ImGui slider bridge) as post-migration polish.
Neither is on the critical path for the dev/bump-genesis-1.0 → main
promotion.

Changes

• src/genelab/scene/interactive_scene.py — two thin wrappers over
  Genesis 1.0's scene.draw_debug_frustum / scene.draw_debug_trajectory:
  • draw_camera_frustums(camera_names=None, color=...) resolves
    names against the scene's CameraSensor set; returns the number
    drawn so callers can detect mis-named selections.
  • draw_camera_trajectory(positions, radius, color) is a verbatim
    pass-through.
    Both raise RuntimeError if called pre-build().
• src/genelab/sensor/camera.py — added a public gs_camera
  property so external debug code reaches the Genesis handle through
  the same surface instead of _cam.
• tests/test_scene_camera_debug.py — 7 new tests via a
  _FakeGsScene that records every debug-draw call; a monkeypatch
  routes isinstance(s, CameraSensor) through fakes so no real
  Genesis env is needed.
• docs/concepts/scene.{en,zh}.md — new Camera debug helpers
  section.
• CHANGELOG.md — ### Added entry.

v1.0 inspection notes (other S5 sub-tasks)

ADR S5 sub-task	v1.0 reality	Decision
Extend Articulation to dispatch on .xacro	Genesis 1.0 already auto-preprocesses .xacro / .urdf.xacro files when loaded via gs.morphs.URDF (the parser is invoked at URDF morph build time). GeneLab's Articulation is still MJCF-only — adding URDF/xacro is a real surface change to ArticulationCfg (parallel urdf_path field + dispatch in spawn() + asset_zoo updates).	Carved to post-migration polish. Users who need xacro today can call gs.morphs.URDF(file="...xacro", xacro_args={...}) directly; promoting it to ArticulationCfg is a follow-up after the migration lands on main.
Add a sample .xacro robot under asset_zoo	Hinges on the ArticulationCfg URDF support landing first.	Carved with the parent sub-task.
draw_camera_frustums() + draw_camera_trajectory()	Genesis 1.0 exposes both via scene.draw_debug_frustum(camera, color) and scene.draw_debug_trajectory(poss, radius, color).	Shipped in this PR.
Wire --debug-cameras CLI flag	Trivial follow-up once the helpers exist; needs an InteractiveSceneCfg.debug_cameras flag and a one-line call from the step hook.	Carved to a future commit on this branch or a separate PR — non-blocking for promotion.
ImGui slider bridge mapping ImGui sliders to joint targets	Genesis's viewer ImGui hook can't be probed headlessly; needs a display-equipped environment to validate the callback API. Existing bridges/dearpygui.py already provides a working twist-bridge (over DearPyGui's own loop), so the ImGui-native equivalent is an additive, not a blocker.	Carved to post-migration polish. The S1-added SimulationCfg.viewer_imgui flag already plumbs Genesis's ImGui overlay; the slider bridge is a separate (display-required) feature.

Test plan

• ruff check
• ruff format --check
• pyright — 0 errors, 0 warnings
• pytest — 605 passed (+7 scene-debug), 0 failed
• mkdocs build --strict

Risks

• Pre-build call sequencing. Both helpers raise if called before
  build(). Play-time hooks (e.g. a custom EnvHook or a
  RecorderBridge extension) need to register after the env's
  build phase — same constraint every sensor wrapper already obeys.
• Camera name resolution is by sensor name, not link name. The
  helpers walk scene._sensors. A scene with multiple cameras
  attached to the same link works fine; mistyping the name surfaces
  with a KeyError listing the available sensor names.

Notes

• After this merges, the remaining S5 sub-tasks are deferred to
  post-migration polish. The integration branch is then ready for the
  promotion PR dev/bump-genesis-1.0 → main. At ~22 commits
  ahead of main, the diff is reviewable in one pass.
• S3.3 (registered tactile example task) is also still parked as
  parallel housekeeping; carving it the same way unless you want it
  before promotion.