feat: ParkedCarStreamer — frustum-gated runtime spawn with pooling

[nigel-hall-codes]

The #202 POC payoff: runtime-spawn parked cars only where the camera can see them.

Scope

• Fork TrafficManager's loop: ring + distance cull + paced maxSpawnsPerTick + wait-for-chunk road raycast + shared target resolution.
• Add the two upgrades it lacks:
  • a real ObjectPool<T> (TrafficManager Instantiate/Destroys);
  • a frustum spawn-gate (GeometryUtility.CalculateFrustumPlanes + TestPlanesAABB).
• Route the gate through a single IsVisible(pos) seam so occlusion drops in later untouched.
• Tune hysteresis/latency so cars don't visibly pop as the camera turns.
• Measure against the spike's baseline.

Blocked by the runtime-data change. Parent: #202.