Add optional rerun debug view of placement validation - #984

- Add debug_visualize / debug_visualize_rrd_path to PlacementValidatorSpec so
  an env's YAML can ask for the Rerun view without editing Python.
- Forward both through build_checks_for_placer_params into ObjectPlacerParams.
- Turn the view on in the butter_raisin_box scene as a worked example.
- Cover the default-off and forwarding paths in the graph-spec tests.

Signed-off-by: Xinjie Yao <xyao@nvidia.com>

- Spawn the Rerun viewer under `setpriv --pdeathsig TERM` instead of `rr.spawn()`, so the
  kernel closes the window when the run exits.
- Isaac Sim's `SimulationApp.close()` ends in `os._exit()`, so no `atexit` hook can do this.
- Stops a stale viewer from holding port 9876, which silently made the next run's spawn a no-op.
- Wait for the viewer to serve before logging, replacing the readiness wait `rr.spawn()` did.

Signed-off-by: Xinjie Yao <xyao@nvidia.com>

- Take `debug_visualize: true` out of the shipped butter/raisin scene, where it spawned a viewer
  window on every build, and move the worked example to a test-data graph YAML.
- Guard that with a test asserting no versioned env under `isaaclab_arena_environments` asks for it.
- Track which candidates each check was run on, so the view no longer draws an expensive check as
  rejecting a layout it skipped; the summary count now derives from the same record.
- Warn when the viewer port is already served, and confirm the spawned viewer is still alive once
  it answers, so a run cannot silently log into somebody else's window.
- Fall back to killing a viewer that ignores SIGTERM instead of raising out of close().

Signed-off-by: Xinjie Yao <xyao@nvidia.com>

- Acceptance now follows the placer, which gates layouts on required_checks alone.
- A failure the placer does not gate on reads as accepted, naming the advisory check.
- Pull the wording out into summarize_candidate_verdict() so the rule is unit-testable.

Signed-off-by: Xinjie Yao <xyao@nvidia.com>

- Show the env graph YAML and Python routes in the visualizer module, pointing at the worked example.
- Say the same from the reachability check, whose layer only appears once that view is on.
- Drop ACCEPTED_COLOR / REJECTED_COLOR, which nothing has ever drawn with.

Signed-off-by: Xinjie Yao <xyao@nvidia.com>

Keep ObjectPlacerParams data-only: checks reach the live Rerun view
through get_active_placement_visualizer() rather than a field on the
params, and the per-batch layout/verdict logging moves off ObjectPlacer
onto the visualizer that owns it.

Rename debug_visualize_rrd_path to debug_visualize_output_path and the
opaque batch "slot" to batch_index, shorten the docstrings that
described code living elsewhere, and fix the enabled check name in the
worked YAML example (ik_reachability -> ik_reachable), which was being
silently dropped as unregistered.

Signed-off-by: Xinjie Yao <xyao@nvidia.com>

Say what each piece does in plain terms: shorter constant docs, one
straightforward line per function, and the remaining rationale (setpriv,
the shared view, the deepcopy guard) stated directly instead of alluded
to.

Signed-off-by: Xinjie Yao <xyao@nvidia.com>