Add physics colliders to buildings and roadside props

[Copilot]

Buildings and roadside props (lamp posts, trees, signs, fences) were purely visual — the car passed through them with no collision response.

Changes

• Building colliders — BuildBuilding() now adds a MeshCollider to both the Walls and Roof child GameObjects:

  wallGo.AddComponent<MeshCollider>().sharedMesh = result.WallMesh;
  roofGo.AddComponent<MeshCollider>().sharedMesh = result.RoofMesh;

• Prop colliders — BuildRoad() calls RoadsidePropPlacer.Place() and passes each result to the new SpawnPropCollider() helper, which creates a collider-only GameObject at each prop's world position with a shape appropriate to its type:

  PropType	Collider	Dimensions
  LampPost, SignPost	CapsuleCollider	r=0.1 m, h=4 m
  Tree	CapsuleCollider	r=0.3 m, h=4 m
  Fence	BoxCollider	2 × 1.5 × 0.1 m

• Tests — new CollisionPlayModeTests verifies collider presence and dimensions for each building part and prop type, and asserts that props are placed outside the drivable road surface.

Original prompt

Add collision to buildings and other props that aren't on the road, so when the car runs into a building, it is prevented from going through.

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[adam133]

@copilot review and fix the merge conflicts

[Copilot]

@copilot review and fix the merge conflicts

Resolved in commit 0c34246. The conflict was in MapSceneBuilder.BuildRoad() — our prop collider spawning block clashed with the ditch mesh rendering block added by PR #62. The resolved version keeps both: lane markings → kerb → ditch → prop colliders, in that order.

[adam133]

@copilot there are still merge conflicts. Rebase these changes onto main and retry the PR

[Copilot]

@copilot there are still merge conflicts. Rebase these changes onto main and retry the PR

The local branch is cleanly rebased on top of main (commit e96ecaa sits directly on 87c2eb9), but pushing it requires a force-push, which I'm unable to do from within this environment. The report_progress tool can't force-push diverged histories either.

To finish this, please run the following from your local machine:

git fetch origin
git push origin e96ecaa:copilot/add-collision-to-buildings --force-with-lease

This will update the PR branch to the cleanly rebased state with only our 2 commits on top of main, and no merge commit. All 653 tests pass on that state.