preload does not block on custom fetch calls before setup runs

Observed behavior

When a sketch uses preload() with loadImage() the setup() is deferred until the image finishes loading. However, a fetch() call placed inside preload() does not pause setup(); the sketch proceeds while the network request is still in flight.

Constraints

• Using p5.js 1.9.x via npm ESM import (import p5 from 'p5') in instance mode.
• Test harness loads the sketch in a headless JSDOM environment where fetch is polyfilled.
• No production credentials are involved; the goal is deterministic initialization order for snapshot tests.

Unresolved decision

The documentation states that preload() only waits for p5’s built-in loaders. It is unclear whether wrapping a fetch() in a Promise and returning it from preload() would be respected, or if the only supported pattern is to move custom async work into setup() and guard draw() with a readiness flag.

Specific questions:

1. Does returning a Promise from preload() actually delay setup() in current releases?
2. If not, what is the recommended pattern for ensuring external data is available before the first draw() without polling in draw()?
3. Are there version-specific differences between the CDN UMD build and the npm ESM build regarding this behavior?