Add grey visual meshes to roadside props

[Copilot]

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.

Original prompt

PR can't be merged because of merge conflicts, and copilot can't fix them without force pushing, which it is not allowed to do.

male the changes in this pr again on a fresh branch. #63

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[adam133]

@copilot trees and signs, lampposts, and fences all have some sort of texture so they are visible right?

[Copilot]

@copilot trees and signs, lampposts, and fences all have some sort of texture so they are visible right?

No — the props are currently collider-only. SpawnPropCollider creates a GameObject with only a physics collider component (no MeshFilter, MeshRenderer, or texture), so lamp posts, trees, sign posts, and fences block the car but are invisible in-game.

There is no procedural prop mesh generator in the codebase today, so adding visible geometry would be a separate feature on top of this collision work. This PR's scope was specifically to prevent the car from passing through objects that are already rendered (buildings) and to add invisible physics barriers at prop positions along the road.