feat: game/engine split refactor + skybox

tests/layering.test.ts

@@ -0,0 +1,32 @@
+import { expect, test } from "bun:test"
+import { readdirSync, readFileSync, statSync } from "node:fs"
+import { join } from "node:path"
+
+// The engine is the reusable, content-agnostic layer: it must import nothing from
+// game (content) or app (browser glue). game -> engine and app -> game -> engine are
+// fine; the reverse is the violation. This freezes the Stage-3 seam so a stray import
+// can't quietly re-couple the layers.
+function tsFiles(dir: string): string[] {
+  const out: string[] = []
+  for (const name of readdirSync(dir)) {
+    const p = join(dir, name)
+    if (statSync(p).isDirectory()) {
+      out.push(...tsFiles(p))
+    } else if (p.endsWith(".ts")) {
+      out.push(p)
+    }
+  }
+  return out
+}
+
+test("engine imports nothing from game or app", () => {
+  const engineDir = new URL("../engine", import.meta.url).pathname
+  const offenders = tsFiles(engineDir).filter((f) => /from\s+["'][^"']*\/(?:game|app)\//.test(readFileSync(f, "utf8")))
+  expect(offenders).toEqual([])
+})
+
+test("game imports nothing from app", () => {
+  const gameDir = new URL("../game", import.meta.url).pathname
+  const offenders = tsFiles(gameDir).filter((f) => /from\s+["'][^"']*\/app\//.test(readFileSync(f, "utf8")))
+  expect(offenders).toEqual([])
+})