chore(vendor): import pi 1.3.0

Imported as a linear vendor commit so forge rebases do not replay the upstream root tree.

git-subtree-dir: third_party/pi

git-subtree-split: 71023aec1546d6d677a65669c5665c0927ce0dd0

third_party/pi/packages/chord/test/fixtures/counting.ts

@@ -0,0 +1,73 @@
+// A link wrapper that counts what crosses it, per direction: frames by type and bytes (JSON text length). It can also
+// hold back the acknowledgements its own side sends (`paused`), to fill the peer's flow-control window.
+import type { Frame, Link } from "../../src/index.ts";
+
+export class CountingLink implements Link {
+	readonly inner: Link;
+	/** Frames this side sent, in order. */
+	readonly sent: Frame[] = [];
+	/** Frames this side received, in order. */
+	readonly received: Frame[] = [];
+	sentBytes = 0;
+	receivedBytes = 0;
+	/** Acknowledgements are held back (the peer's window fills up). */
+	paused = false;
+	#held: Frame | undefined;
+
+	constructor(inner: Link) {
+		this.inner = inner;
+	}
+
+	get closed(): boolean {
+		return this.inner.closed;
+	}
+
+	send(frame: Frame): number | undefined {
+		if (frame.t === "ack" && this.paused) {
+			this.#held = frame;
+			return 0;
+		}
+		this.sent.push(frame);
+		this.sentBytes += JSON.stringify(frame).length;
+		return this.inner.send(frame);
+	}
+
+	/** Lets the held acknowledgement through and stops holding them. */
+	release(): void {
+		this.paused = false;
+		const held = this.#held;
+		this.#held = undefined;
+		if (held !== undefined) this.inner.send(held);
+	}
+
+	receive(handler: (frame: Frame) => void): void {
+		this.inner.receive((frame) => {
+			this.received.push(frame);
+			this.receivedBytes += JSON.stringify(frame).length;
+			handler(frame);
+		});
+	}
+
+	onClose(handler: (reason: string) => void): void {
+		this.inner.onClose(handler);
+	}
+
+	close(reason?: string): void {
+		this.inner.close(reason);
+	}
+
+	/** Frames of type `t` this side sent (`sent`) or received (`received`). */
+	count(direction: "sent" | "received", t: Frame["t"]): number {
+		return this[direction].filter((frame) => frame.t === t).length;
+	}
+
+	/** Frames this side sent or received, except pings, pongs and acknowledgements. */
+	traffic(direction: "sent" | "received"): Frame[] {
+		return this[direction].filter((frame) => frame.t !== "ping" && frame.t !== "pong" && frame.t !== "ack");
+	}
+}
+
+export type StateFrame = Extract<Frame, { t: "state" }>;
+
+export const stateFrames = (frames: readonly Frame[]): StateFrame[] =>
+	frames.filter((frame): frame is StateFrame => frame.t === "state");