test: real shader/terrain tests + headless render check

npm test was a brace counter: it checked each file was >10 chars and that {
and } counts matched. It passed on a build whose shader would not compile.

- tests/shaders.test.js compiles the game's real GLSL with glslangValidator
  (Khronos' reference compiler) under a three.js-style attribute prefix, and
  includes a canary that asserts a known-bad vec4 construction is REJECTED, so
  the suite cannot pass vacuously.
- tests/terrain.test.js covers heightfield determinism (the "persistent world"
  claim), airport flattening inside the 200 m radius with no leakage outside it,
  out-of-bounds behaviour, plausible height range, and vertex colour counts.
- scripts/headless-check.mjs (npm run test:render) serves dist/, drives the
  game over raw CDP with SwiftShader software WebGL, and fails if the canvas is
  black, the shaders fail to compile, or the scene does not change after input.
  No new dependencies: Node 22 built-in fetch/WebSocket. 120 s watchdog so it
  cannot hang a runner.
- npm test -> node --test "tests/*.test.js"; old checker kept as npm run lint:syntax.
- CI installs glslang-tools and runs the real tests; render job is
  continue-on-error because not every runner has a Chrome binary.

Verified non-vacuous: re-breaking the shader makes the unit tests fail 2/4 with
glslang's own "constructor: too many arguments" and makes the render check fail
four ways (1 distinct colour, 0/64000 lit pixels, shader error, frozen scene).

package.json

@@ -12,6 +12,9 @@
   "scripts": {
     "dev": "vite",
     "build": "vite build",
-    "test": "node scripts/check-syntax.js"
+    "preview": "vite preview",
+    "test": "node --test \"tests/*.test.js\"",
+    "test:render": "node scripts/headless-check.mjs",
+    "lint:syntax": "node scripts/check-syntax.js"
   }
 }