Strip unused madrona subsystems (physics, navmesh) from the batch renderer

[duburcqa]

Aggressively remove everything the Genesis batch-render path doesn't use, in one PR (~8.4k lines deleted). Success criterion: the ~37 batch-render unit tests (rasterizer + raytracer) stay green.

Removed

• Rigid-body physics simulation (~7.4k lines). Genesis runs its own physics and feeds transforms to the renderer, so madrona's solvers are never added to the render task graph (sim.cpp sets up only rendering). Deleted xpbd, tgs, narrowphase, broadphase, physics.cpp, physics_assets, physics_loader and their headers; dropped them from the JIT megakernel sources and the bridge links; removed the physics CMake subdirectory.
• navmesh. madrona_navmesh was never linked by any target and no ECS system referenced it — pure dead weight in the megakernel. Removed the library, the megakernel source, and the files.
• CPU multi-world executor. The renderer runs GPU-only (ExecMode::CUDA / MWCudaExecutor); madrona_mw_cpu (cpu_exec.cpp + mw_cpu headers) is never instantiated. Removed the library, its bridge link, and the vestigial mw_cpu.hpp include.

Kept (the render path needs it)

The mesh-BVH / geometry infrastructure the raytracer depends on: mesh_bvh.{hpp,inl}, geo.{hpp,inl,cpp}, gjk.hpp. mesh_bvh_builder.cpp now includes mesh_bvh.hpp directly instead of physics_assets.hpp (it only needed MeshBVH).

Validation

• Host build: green.
• The device megakernel is JIT-compiled at runtime, so the render path is validated by the batch-render tests on the cluster (genesis Production CI). Each removed subsystem was verified to be absent from the render task graph before deletion, so rendering should be unaffected.

Builds on the v0.0.9.post1 render-crash fix already on main.