Master materials

A master material defines the features and the exposed parameters for a family: painted metal, terrain, skin, glass, and so on. Instances do not copy the graph.

JRV-0051 has in-memory masters, not disk assets. MaterialLibrary::add_master validates the graph, lowers it, and compiles it once. The master keeps the graph, the IR, and the compiled program, plus a revision. A later graph edit is supposed to bump that revision and recompile. Hot reload is not built.

The presented master is standard_metal_rough: Surface / StandardMetalRough / Opaque. Its schema is BaseColorFactor, MetallicFactor, RoughnessFactor, NormalScale, OcclusionStrength, EmissiveFactor, plus texture parameters BaseColor, Orm, Normal, Emissive, and sampler MaterialSampler. Scalars are still one vec4 slot each. The master does not require ORM. The graph inside this master chooses that packing. An Unlit master (Tint, BaseTexture) still compiles. Static switches are documented and not implemented. Names are for authoring. The packed bytes are what the renderer uploads.

Phase 0 has a single jarvig.material/v1 document, not a parent/child link. That document is the stand-in for "the thing an instance will point at." JRV-0036 adds inheritance: scalar, vector, texture, enum, and limited static feature overrides without rebuilding the graph.

Do not store a second full graph on an instance to fake inheritance. The acceptance test is that a parameter-only edit does not recompile unrelated graph structure.