[Materials] Add a shared energy-conserving layered PBR model

[proggeramlug]

Parent: #126
Inventory: #11 material/BRDF section

Problem

The current PBR path covers core metallic/roughness rendering, but UE-class look development needs composable, energy-conserving material lobes rather than one-off shader forks. glTF extensions such as clearcoat, sheen, anisotropy, iridescence, specular, and IOR are also common interoperability requirements.

Outcome

A versioned layered-PBR material model shared by deferred/visibility, forward transparent, path-traced, and reference-render validation paths.

Minimum first release:

• base metallic/roughness dielectric/conductor;
• clearcoat with independent normal/roughness;
• sheen;
• tangent-space anisotropy;
• specular/IOR controls;
• iridescence/thin-film lobe;
• emissive energy represented consistently for exposure and GI.

Screen-space subsurface profiles may be added as a separate milestone only after the core lobe ABI is stable.

Material ABI

• Version the GPU material record; declare exact alignment/packing once for Rust and WGSL.
• Use a lobe/feature bitmask plus indices into texture/resource tables.
• Centralize BRDF functions in shared WGSL included by every relevant pipeline.
• Preserve a compact fast path for common base-only materials; do not branch every pixel through all lobes if no lobe is active.
• Translate glTF extensions without losing texture transforms, factors, color spaces, or tangent requirements.
• Invalid/missing tangent data must select a documented fallback or emit an actionable import warning.

Physical correctness rules

• Enforce energy compensation between base specular/diffuse and added lobes.
• Use consistent normal orientation, geometric normal safeguards, and roughness floors.
• Filter normal variance for minification; do not let added lobes reintroduce sparkle/aliasing.
• Match Fresnel/IOR conventions across realtime and reference/path-traced implementations.
• Explicitly define multiple-scattering approximation and reciprocity assumptions.

Implementation sequence

1. Write a CPU/reference evaluation and sphere-matrix corpus for the base model.
2. Introduce versioned lobe ABI without visual change.
3. Land clearcoat + IOR/specular and validate energy.
4. Land sheen and anisotropy with correct tangent handling.
5. Land iridescence with bounded parameter ranges.
6. Wire import, serialization, authoring API, debug views, and path-tracer parity.

Each lobe should be a reviewable commit/PR slice with its own tests even if one agent owns the whole issue.

Acceptance criteria

• A generated parameter matrix compares each lobe against tools/bloom-reference or Cycles at multiple view/light angles; thresholds and known model differences are recorded.
• White-furnace tests show no unexplained energy gain across roughness, metallic, IOR, and lobe-weight sweeps.
• Deferred/visibility, forward, and path-traced versions agree within the approved model tolerance.
• Canonical glTF extension assets import with correct factors/textures and no silent fallback.
• Anisotropy orientation follows mesh tangents and survives negative scale/mirrored UV cases.
• Normal-map minification does not create temporal sparkle in the motion corpus.
• Base-only material cost does not regress by more than an approved measured budget; attach shader stats and pass timings.
• Public APIs remain simple: useful defaults and one material descriptor, not a required sequence of low-level FFI calls.

Likely files

• native/shared/src/renderer/material_pipeline.rs, material_system.rs, shaders/
• native/shared/shaders/material_abi.wgsl
• native/shared/src/models_gltf.rs
• tools/bloom-reference/src/tracer.rs
• path-tracer shaders under native/shared/src/renderer/shaders/pt.rs
• src/models/ public material types

Non-goals

• A node-based material editor.
• Strand hair, eye geometry, or a full BSSRDF in the first release.
• Maintaining two mathematically unrelated BRDF implementations.

Dependencies

• [Renderer] Add capability-tiered global material and texture indirection #130 for scalable texture references.
• [Materials] Implement correct glTF alpha blending, transmission, volume, and refraction #133 for lobe use on transmitted/transparent materials.