Function render_preview_html 

pub fn render_preview_html(receipt: &SessionReceipt) -> String

Generate a self-contained preview.html that embeds the receipt JSON and runs Merkle verification client-side using Web Crypto API.

The HTML works fully air-gapped: no network calls, no CDN, no server. Open it in any modern browser and it automatically verifies the receipt and shows pass/fail for each check.