Add PBRT v4 scene importer

[tarcila]

Parses .pbrt files (including Include/Import with cycle detection) and maps them onto TSD's ANARI-mirrored scene.
Built around a stream Lexer, a recursive-descent Parser preserving PBRT v4 typed-parameter qualifiers ("rgb", "spectrum", "blackbody", "texture", "float", ...), and a scene walker (import_PBRT.cpp) that emits TSD geometries, materials, lights and a camera. Scene-relative file references are sandboxed (resolveScenePath rejects absolute paths). ObjectInstance templates are built once and shared across instances.

Supported

• Directives: WorldBegin/End, AttributeBegin/End, TransformBegin/End, ObjectBegin/End, ObjectInstance, MediumInterface, MakeNamedMedium, ReverseOrientation, Option, all transform directives (Identity, Transform, ConcatTransform, Translate, Scale, Rotate, LookAt, CoordinateSystem, CoordSysTransform), Include/Import.
• Shapes: trianglemesh, plymesh (tinyply, with N / uv / vertex_index variants), sphere, cylinder, disk (with annulus and phimax), curve (bezier and bspline; 4 control points), bilinearmesh (triangulated), loopsubdiv (passed through as the control mesh).
• Materials: diffuse, coateddiffuse, conductor, dielectric, thindielectric, diffusetransmission, coatedconductor, interface, mix (full RGB lerp; texture-driven amount baked into the mask sampler for baseColor).
• Textures: imagemap (with sRGB vs linear selection, uscale/vscale/ udelta/vdelta UV transform, grayscale-to-RGB broadcast via outTransform), constant, scale chains (PBRT v4 'tex'/'scale' slots, affine-baked into the sampler), mix of constant + textured.
• Normal maps: explicit "normalmap" path, plus a fallback that converts a "texture displacement" height map into a tangent-space normal via central differences.
• Per-shape alpha: float cutoff or texture (alphaMode = mask), with baseColor.w neutralised so RGBA leaves don't double-apply alpha.
• Lights: distant, point, spot, infinite (HDRI; equal-area sphere maps auto-converted to equirectangular). Area lights become emissive physicallyBased materials. Emission values honour "rgb"/"color"/"xyz", "blackbody" (Tanner Helland approximation), and sampled-spectrum inputs (averaged), with the PBRT "scale" parameter and film ISO applied as exposure.
• Volumes: homogeneous media via KHR_materials_volume (attenuationColor = exp(-sigma_a), attenuationDistance = 1) on dielectric / thindielectric / interface.
• Camera: perspective (with portrait-aware fov->fovy conversion), orthographic, depth of field via lensradius/focaldistance, shutter window.

Approximations and limitations

• Geometric displacement is not real displacement — converted to a tangent-space bump-normal map (kBumpStrength=16) since ANARI has no equivalent.
• Ribbon curves rendered as cylinder curves; only 4 control points are accepted per curve.
• ReverseOrientation is only honoured on trianglemesh, plymesh and disk; other shape types log a warning.
• 'scale' or 'mix' of two image textures collapses to a single sampler with the second's tint applied (ANARI samplers can't chain).
• 'mix' material can only carry a texture-driven amount on baseColor; metallic/roughness/specular/clearcoat/transmission/ior collapse to a uniform lerp at the chosen amount.
• Sampled spectra are approximated as the mean of their sample values (grayscale); named spectra (e.g. "spectrum L" "stdillum-D65") are unsupported and fall back to a neutral default.
• Dielectric IOR is capped at 2.5 — single-scalar IOR plus ANARI volumes can't reproduce PBRT's spectral path tracing at higher eta.
• Conductor 'eta' "metal-*-eta" named spectra are mapped to fixed normal-incidence RGB approximations (Cu/Au/Ag/Fe/Cr/Ni/Ti/Pt, default aluminium); roughness textures are supported.
• subsurface, measured and hair materials are heuristic stand-ins (white glossy / matte) — no SSS, no tabulated BSDF playback, no hair scattering.
• Heterogeneous media, emissive media, and sigma_s scattering are unsupported; scattering coefficients are folded into absorption with a warning.
• Cameras: spherical and realistic types are downgraded to perspective.
• Integrator, Sampler, PixelFilter, ColorSpace, Accelerator, ActiveTransform and TransformTimes are parsed and discarded — no motion blur, no spectral integration, no custom samplers.
• The animation manager parameter is unused; no keyframe data is emitted.

[jeffamstutz]

LGTM, thanks!