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/mcp/test/fixtures/stubborn-server.mjs

@@ -0,0 +1,22 @@
+// Answers initialize, spawns a grandchild that outlives stdin, and ignores stdin EOF and SIGTERM.
+import { spawn } from "node:child_process";
+import { createInterface } from "node:readline";
+
+process.on("SIGTERM", () => {});
+const grandchild = spawn(process.execPath, ["-e", "process.on('SIGTERM', () => {}); setInterval(() => {}, 1000)"], {
+	stdio: "ignore",
+});
+console.error(`grandchild ${grandchild.pid}`);
+setInterval(() => {}, 1000);
+
+const lines = createInterface({ input: process.stdin, crlfDelay: Infinity });
+lines.on("line", (line) => {
+	const message = JSON.parse(line);
+	if (message.method !== "initialize") return;
+	const result = {
+		protocolVersion: "2025-06-18",
+		capabilities: {},
+		serverInfo: { name: "stubborn-fixture", version: "1.0.0" },
+	};
+	process.stdout.write(`${JSON.stringify({ jsonrpc: "2.0", id: message.id, result })}\n`);
+});