perf: compile district shaders at the on-ramp instead of mid-drive

src/scene/world/districts.ts

@@ -246,6 +246,9 @@ export function createDistricts(
     );
 
   const dummy = new THREE.Object3D();
+  // Draw freshly installed (still flat) pieces for a couple of frames so their
+  // shader programs compile while parked at the on-ramp, not mid-drive.
+  let warmFrames = 0;
   const install = (piece: DistrictPiece, body: Geometry, light: Geometry) => {
     const entry = state.get(piece);
     if (!entry) return;
@@ -275,6 +278,7 @@ export function createDistricts(
       mesh.instanceMatrix.needsUpdate = true;
       entry.meshes.push(mesh);
       group.add(mesh);
+      warmFrames = 2;
     }
   };
 
@@ -303,6 +307,8 @@ export function createDistricts(
     group,
     update(distance, energy, section, elapsed, dt) {
       current = section;
+      const warming = warmFrames > 0;
+      if (warming) warmFrames -= 1;
       shared.uDistance.value = distance;
       shared.uEnergy.value = energy;
       shared.uTime.value = elapsed;
@@ -313,7 +319,7 @@ export function createDistricts(
         const rate = target > entry.rise.value ? 0.9 : 1.4;
         entry.rise.value += (target - entry.rise.value) * (1 - Math.exp(-dt * rate));
         if (Math.abs(target - entry.rise.value) < 0.002) entry.rise.value = target;
-        const visible = entry.rise.value > 0.002;
+        const visible = warming || entry.rise.value > 0.002;
         for (const mesh of entry.meshes) mesh.visible = visible;
       }
     },