rendering: preserve material and lighting semantics in DrawList

[tritao]

Summary

This layer retains and executes the ordinary visual semantics that sit above the minimal DrawList executor:

• material fields: ambient, diffuse, specular, emissive, shininess, opacity, and shading model;
• directional, point, and spot lights, attenuation, two-sided lighting, and the explicit eight-light Visual-program cap;
• texture pixels, wrapping, effective filtering, mipmap policy, texture models, and blend color;
• depth, blend, alpha-test, transparency, and per-material primitive batching;
• composable GLSL material, texture, lighting, and alpha-test modules.

Traversal state remains semantic and renderer-neutral. The OpenGL executor owns backend fallbacks and diagnostics: missing lighting never synthesizes a headlight, dual-source blend factors are retained and warned/fallback only at execution, and retained lights beyond the Visual-program limit are reported rather than discarded from the IR.

The layer also removes the speculative SoShape material-index virtual, preserves diffuse and emissive independently, composes texture evaluation with lighting, and derives sampler state from the existing quality policy.

Scope

This change does not add raster-width/point-size behavior, pixel image/text programs, picking, selection, manager orchestration, render stages, or context-loss lifecycle. Those remain in the layers above this one.

Validation

The retained semantic and core-profile EGL pixel tests cover material, emissive, lighting, texture models, filtering, alpha, depth, blend, two-sided lighting, light-cap behavior, batching, and transparent depth-write behavior.

Stack

Layer 13/18, based on PR #29.