Phase 1: road network and heat

Roads are a jittered 7x7 grid with ~a quarter of the edges thinned out, kept
connected. The loops are the point: "take a different road" is not a decision
without alternative routes.

Every metre driven adds heat to that segment; all roads shed it slowly. Crossing
a threshold escalates Clear -> Patrol -> Barricade -> Turret, each of which puts
real obstructions on the tarmac. Hysteresis stops a road on a boundary from
rebuilding its barricade every few seconds.

Level changes are deferred while the car is on the segment. Building a barricade
around the player would spawn a static collider inside the chassis, and the
player is meant to discover escalation by returning to a road, not by watching
it assemble behind them.

Checkpoint positions are seeded from the segment id, so a stretch of road always
fortifies in the same place — recognising it is part of learning the map.

Scenery now avoids the tarmac and the car spawns at a junction.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/ui/hud.ts

@@ -1,4 +1,5 @@
 import type { CarCondition } from '../sim/car';
+import type { HeatLevel } from '../sim/heat';
 
 const BAR_WIDTH = 12;
 
@@ -7,12 +8,19 @@ function bar(value: number): string {
   return '█'.repeat(filled) + '·'.repeat(BAR_WIDTH - filled);
 }
 
+export interface HeatReadout {
+  segmentId: number | null;
+  value: number;
+  level: HeatLevel | null;
+  hottest: number;
+}
+
 export function createHud(seed: number) {
   const el = document.getElementById('hud')!;
   let last = 0;
 
   return {
-    update(speedMs: number, condition: CarCondition, now: number) {
+    update(speedMs: number, condition: CarCondition, now: number, heat: HeatReadout) {
       // The HUD is debug scaffolding, not the real interface — 10 Hz is plenty.
       if (now - last < 0.1) return;
       last = now;
@@ -23,6 +31,12 @@ export function createHud(seed: number) {
         `tires   ${bar(condition.tires)} ${(condition.tires * 100).toFixed(0)}%`,
         `chassis ${bar(condition.chassis)} ${(condition.chassis * 100).toFixed(0)}%`,
         '',
+        // Debug only. The real game signals heat through the road itself —
+        // see the design brief: no numeric meter ships.
+        `[debug] road    ${heat.segmentId === null ? 'off-road' : `#${heat.segmentId}`}`,
+        `[debug] heat    ${bar(heat.value)} ${heat.level ?? '—'}`,
+        `[debug] hottest ${bar(heat.hottest)}`,
+        '',
         `seed ${seed}`,
         'WASD drive · space handbrake · R respawn',
       ].join('\n');