p6-f2: imported prop textures — UVs and base-colour images through PropPart, the batch path and both exporters

[JArmandoAnaya]

Split out of #322 (p6-s8) · governed by ADR-0013.

Problem

p6-s8 imports a glTF/GLB as a prop, but an imported model's textures are
discarded. props::PropPart carries positions, normals, indices and a single
flat linear-RGB color — no UVs, no texture reference
(core/include/roadmaker/assets/prop_library.hpp:40-46). So the importer decodes
the primitive's baseColorTexture, averages it, multiplies by
baseColorFactor, and stores that one colour, warning per primitive as it goes.

A user importing a patterned chair gets a chair in roughly the right colour. That
is honest, and it is the decision ADR-0013 records — but it is not the feature.

Why it was not done in p6-s8

Because PropPart is not the only thing in the way. Every consumer of a prop
model assumes flat colour:

• editor/src/viewport/viewport_widget.cpp:625 — all props draw with one
  shared material_for(SurfaceKind::Untextured, {}).
• editor/src/render/scene_builder.cpp:245-249 — a batch converts each part to
  RenderMeshData once, colour only; RenderMeshData does have a uvs field
  (editor/src/render/renderer.hpp:115-120), so the renderer is not the blocker.
• core/src/io/gltf_exporter.cpp — one glTF material per (model_id, part.name)
  via mesh_export_common.hpp:130, flat colour.
• core/src/io/usd_exporter.cpp:63-71 — its own local MaterialDef{color3, roughness}.

Doing it properly is a change to the kernel representation, the instanced upload
path, and both exporters — which crosses the export byte-identity surface.
That is its own sprint, not a rider on an import sprint.

Scope

• PropPart gains std::vector<double> uvs and an optional base-colour image
  (owned by the model, so props::model()'s pointer contract is unchanged).
• The importer stops flattening: it keeps UVs and the decoded image, and the
  warning this issue is cited from disappears.
• ScenePropBatch carries the material; the viewport uploads it once per batch
  (the sign-face path at viewport_widget.cpp:179-197 is the precedent for a
  runtime-generated texture) and stops sharing one untextured material.
• The glTF exporter embeds the image; the USD exporter emits a UsdUVTexture.
• Existing bundled props have no UVs and must keep exporting byte-identically
  — that is the regression gate, not a nice-to-have.

Acceptance

• A textured GLB imports, renders with its texture, and exports with it to both
  .glb and .usda.
• Every bundled prop's .glb and .usda output is byte-identical to before.
• No import warning about flattening remains.