Function bounded_fetch_range 

pub async fn bounded_fetch_range(
    url: String,
    start: u64,
    end_exclusive: u64,
    max_response_bytes: u64,
    cancellation: &BrowserRangeAbort,
) -> Result<Uint8Array, JsValue>

Executes one strictly bounded browser HTTP Range fetch.

The server must return 206, an exact Content-Length, and a matching response length. No bytes are copied into WASM until those headers pass.