Two adjacent hygiene defects in the Isaac backend's renderer configuration

1. render_mode is accepted but never selects a renderer

create_simulation("isaac", render_mode="rtx_realtime" | "rtx_pathtracing") stores the value and gates a few code paths (headless errors, warmup, native-res upscale), but SimulationApp is constructed with only {"headless": ...} - isaac/simulation.py:989 -> _get_or_create_simulation_app :358-363. Neither value ever changes the active renderer; the robot pass always runs default RTX real-time with default post-processing. A rtx_pathtracing request is silently a no-op renderer-wise - the "silently-dropped kwargs" pattern the review learnings forbid.

Fix options (pick one, explicitly): forward a renderer selection into the SimulationApp launch config / carb settings, or narrow the parameter's documented semantics to what it actually does and validate values against that.

2. _configure_renderer and the backend _add_lighting are dead code

• _configure_renderer (isaac/simulation.py:6155-6184): defined, never called anywhere in the tree; only referenced by a comment at :5127 that reads as if it were active. It carries the DLSS/TAA/denoiser mitigation carb settings - so the mitigation half of the documented DLSS-ghosting strategy is not actually applied (its own docstring at :6158-6167 notes RTX re-asserts DLSS every tick, which may be the reason it was abandoned - if so, delete it and say that where the comment points at it).
• Backend _add_lighting (:6186-6215): defined, never called; the only _add_lighting actually executed is the example's own (examples/isaac_gs/scene.py:79, called at :250).

Repo rule 10: no dead code - wire them or delete them, and correct the :5127 comment either way.

Relation

Surfaced while diagnosing photorealism of examples/isaac_gs (#2321, #2322, #2323); the harmonization issue assumes a decision here about whether robot-pass render settings become configurable.

Board routing: please add to Strands Labs - Robots with Status=Todo, Priority=Low (filing account lacks org project write).