[Investigation] `Document-Isolation-Policy` as a primary embedding lane

Today, embedding Kandelo on a third-party page requires the embedder to opt into COOP/COEP, which is a major adoption blocker. The `Document-Isolation-Policy` (DIP) header lets an iframe declare its own isolation and obtain `SharedArrayBuffer` without parent COEP cooperation. It shipped stable in Chrome 137 in May 2025, so on Chrome desktop this is a real, working path today, not a future spec.

#### Why this matters

- Removes the single biggest reason a host site refuses to embed Kandelo.
- Aligns Kandelo's embed story with what WordPress Playground would need to adopt it.
- Falls back cleanly: DIP iframe → COEP-cooperative iframe → popup launcher.

#### Proposed validation

1. Serve `/embed.html` with `Document-Isolation-Policy : isolate-and-credentialless`, no parent COEP.
2. Confirm crossOriginIsolated === true inside the iframe.
3. Boot the kernel worker + a process worker and run hello.wasm.
4. Verify behavior under Safari and Firefox. Fails.

#### Browser support reality

- Chrome desktop 137+: shipped stable.
- Safari: WebKit [issue #399](https://github.com/WebKit/standards-positions/issues/399)
- Firefox: [issue #1074](https://github.com/mozilla/standards-positions/issues/1074).

#### Suggested follow-up if validation passes

Design a `` web component with an auto-degrade ladder [ DIP → COEP cooperative → popup ], so embedders get the best lane their browser supports without per-site config.

Start with /embed.html and the existing kernel worker and process worker boot path. Serve the page with Document-Isolation-Policy and no parent COEP, then check crossOriginIsolated and run hello.wasm. Done means the DIP iframe path is validated on Chrome and its failure behavior is recorded for Safari and Firefox.

Written by the indexing model from the issue text.