A rejected window.__hf.buildReady hold is ignored: check passes, render exits 0 (also with --no-best-effort), nothing is logged (0.8.157) #5603

The three.js adapter docs recommend registering a promise on window.__hf.buildReady for setup the
runtime cannot see (building meshes, compiling shaders, baking data). When that promise rejects, the
runtime treats the hold as released: the render runs every frame, writes the file, reports "Render
complete" and exits 0. The rejection reason is not logged anywhere. snapshot saves frames and check
reports 0 errors.

In a real three.js film the scene was never built, so every frame is empty or broken, and a pipeline that
trusts the exit code ships it. We hit this with WebGPU: with --no-browser-gpu three silently falls back
to WebGL 2, our build check rejects the hold, and the render still exits 0 with a blank 42 s film.
(data-requires-webgpu on the root does fail the render, exit 1 - but only for that one cause.)

Minimal repro

index.html (no dependencies):

<!doctype html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=1920, height=1080" />
    <script>
      window.__hf = window.__hf || {}; window.__hf.buildReady = window.__hf.buildReady || {};
      window.__hf.buildReady.scene = new Promise((res, rej) => { window.__sceneReady = res; window.__sceneFail = rej; });
    </script>
    <style>
      html, body { margin: 0; width: 1920px; height: 1080px; background: #000; }
      #c { position: absolute; inset: 0; width: 1920px; height: 1080px; }
    </style>
  </head>
  <body>
    <div id="root" data-composition-id="main" data-start="0" data-duration="2"
         data-width="1920" data-height="1080" data-no-timeline>
      <canvas id="c" width="1920" height="1080"></canvas>
    </div>
    <script>
      const ctx = document.getElementById("c").getContext("2d");
      // the build fails (here: bad baked data); the hold rejects with a reason
      try { JSON.parse("{not json"); window.__sceneReady(); }
      catch (err) { window.__sceneFail(new Error("build failed: " + err.message)); }
      addEventListener("hf-seek", (e) => {
        ctx.fillStyle = "#e0893c"; ctx.fillRect(0, 0, 1920, 1080);
        ctx.fillStyle = "#000"; ctx.font = "120px sans-serif"; ctx.fillText("t = " + e.detail.time.toFixed(2), 200, 600);
      });
    </script>
  </body>
</html>

npx hyperframes@0.8.157 check                          # 0 errors, 0 warnings, exit 0
npx hyperframes@0.8.157 render --no-best-effort -o out.mp4
echo $?                                                # 0, "Render complete", 2.0 s video

The log contains no line with "build failed", "reject" or "buildReady".

(If the build fails because an asset 404s, check does report http_error, but render still exits 0.
The repro above uses a failure with no network error, which is the common case: a parse error, a missing
GPU feature, a shader that does not compile, a font that does not load.)

Expected

A rejected build hold fails render and snapshot with a non-zero exit and the rejection reason in the
output (at least under --no-best-effort; arguably always - there is no useful output from a composition
that says its own build failed). check could report it as an error too, since it loads the page.

Environment

hyperframes 0.8.157 and 0.8.156 · macOS 26 (Darwin 25.6), Apple M1 Max · Node 26.9.0 ·
chrome-headless-shell 152.0.7977.30 · hardware GPU and --no-browser-gpu both affected.

Workaround we use

Wrap the seek handler and paint a loud colour when the build failed, then check rendered frames for it:
if (!built) { clear to magenta; return; }.