/**
 * Patch `globalThis.fetch` to advertise HTTP/2 in TLS ALPN, with transparent
 * HTTP/1.1 fallback when the server doesn't negotiate `h2`.
 *
 * Bun's HTTP/2 client is gated on `BUN_FEATURE_FLAG_EXPERIMENTAL_HTTP2_CLIENT`,
 * read by the native runtime before any JS executes; assigning to
 * `process.env` from inside JS is a no-op. Per-request `protocol: "http2"`
 * activates h2 over TLS ALPN and rejects with `error.code === "HTTP2Unsupported"`
 * if the server picks anything else, so we catch and retry without the hint.
 *
 * Some HTTPS endpoints (e.g. corporate API gateways behind reverse proxies)
 * advertise h2 via ALPN but then refuse or reset the connection at the HTTP/2
 * framing layer. Bun surfaces these as `ConnectionRefused`, `ConnectionReset`,
 * or `ConnectionClosed` rather than `HTTP2Unsupported`, so we treat those
 * codes as h2-fallback triggers as well.
 *
 * Bun negotiates h2 via ALPN over TLS only (no h2c), so plain `http://` URLs
 * skip the attempt entirely — avoids the throw/retry round-trip for localhost.
 *
 * Idempotent.
 */
export declare function installH2Fetch(): void;