ariadne: port the volume bunny demos (volren + volslice) to DSL via a mesh-SDF volume source

[transfix]

Ariadne (.ari) ports of the cvcGL volume-raycast (volren_bunny) and volume-slice (volslice_bunny) demos — the "finish the Ariadne demo versions, start with the bunnies" work — plus the enabling capability and reusable control components.

The blocker & the fix. These demos had no .ari because the realizer could only load a volume file, while the demos synthesize the volume as the signed distance field of the bunny mesh at runtime. New mesh-SDF volume source in the scene schema:

- node: bunny_volume
  type: volren        # or volslice
  source: { sdf: { mesh: stanford.bunny, dim: 64 } }
  fit: { height: 100, up: y }

The realizer loads the mesh (applying the node's fit: to stand/scale it, for framing parity with a geometry node) and runs cvc::sdf over an N³ grid — so any demo can turn a mesh into a volume with no asset file, pure-DSL on the stock ariadne_hello host. Guarded on CVC_ENABLE_SDF (warns + skips without it). The file-source path is unchanged.

Ports. volren_bunny.ari (distance-field isosurface) and volslice_bunny.ari (a transfer function around the zero level set), both verified rendering headless via EGL (the bunny raycast / sliced on the ground plane).

Reusable components (the "shared functionality" ask). Both demos import the existing stage_lighting + camera_controls components (shared with bunny_shadow), plus two new catalog components — volren_controls and volslice_controls — binding the renderer's two-way cvc::volren/cvc::volslice::state_settings keys (steps/ambient/supersample/shadows; quality/near-plane/max-planes), so the panels are declarative with no host C++. Added both to the component INDEX.

Tests. AriadneScene.VolumeSdfMeshSource (loader parses source: { sdf }); the existing cvcgl_ariadne_realize volren/volslice path is unchanged and green.

Follow-ups: parametrize the volume-control components by node id (the catalog already models args: { node: … }) instead of the bunny_volume convention; extend SceneVolRen with the richer volumetric-shadow/AO/hemisphere knobs for full volren panel parity.