Add grey visual meshes to roadside props - #64

Roadside props (lamp posts, trees, sign posts, fences) were collider-only — invisible in-game despite blocking the vehicle.

Changes

• MapSceneBuilder — SpawnPropCollider made non-static; adds a child Mesh GameObject per prop with a MeshFilter (Unity built-in primitive, scaled to match the collider) and a MeshRenderer applied via the existing Registry:

  // LampPost/SignPost → thin capsule (r=0.1, h=4 m), mid-grey
  AddCapsuleVisual(go, scale: new Vector3(0.2f, 2f, 0.2f), centerY: 2f, textureId: "prop_lamppost");
  // Tree → wider capsule (r=0.3, h=4 m), olive green
  AddCapsuleVisual(go, scale: new Vector3(0.6f, 2f, 0.6f), centerY: 2f, textureId: "prop_tree");
  // Fence → flat box (2×1.5×0.1 m), weathered wood brown
  AddBoxVisual(go, scale: new Vector3(2f, 1.5f, 0.1f), centerY: 0.75f, textureId: "prop_fence");

• PlaceholderMaterialFactory — adds prop_lamppost, prop_signpost, prop_tree, prop_fence to KnownIds with distinct placeholder colours (mid-grey, olive green, weathered-wood brown); picked up automatically by FillMissing at startup.

• PlaceholderMaterialFactoryTests — new TestCase entries and three focused colour-assertion tests for the four new IDs.

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