feat(backend): PiProcessManager (one Pi per session, cap, set_model/thinking)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/test/pi-process-manager.test.ts

@@ -0,0 +1,28 @@
+import { test, expect } from "vitest";
+import { spawn } from "node:child_process";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import { PiProcessManager } from "../src/pi/pi-process-manager.js";
+import { loadConfig } from "../src/config.js";
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+const FAKE = path.resolve(__dirname, "../../harness/tests/fake_pi/fake_pi_rpc.mjs");
+const SCRIPT = path.resolve(__dirname, "../../harness/tests/fake_pi/scripts/f1_disambiguation.json");
+
+test("spawnFor avvia un runtime e il bridge emette il widget F1", async () => {
+  const cfg = loadConfig({ THT_HARNESS_DIR: "../harness" });
+  const mgr = new PiProcessManager(cfg, { spawnFn: () => spawn("node", [FAKE, SCRIPT]) as any });
+  const rt = await mgr.spawnFor("2026-06-27-100000-x", {});
+  const widget = await new Promise<any>((res) => rt.bridge.onClientEvent((e) => e.type === "ui_request" && res(e)));
+  expect(widget.ui_request.widget).toBe("select");
+  mgr.teardown("2026-06-27-100000-x");
+  expect(mgr.count()).toBe(0);
+});
+
+test("oltre maxPiProcesses solleva errore", async () => {
+  const cfg = { ...loadConfig({}), maxPiProcesses: 1 };
+  const mgr = new PiProcessManager(cfg, { spawnFn: () => spawn("node", [FAKE, SCRIPT]) as any });
+  await mgr.spawnFor("a", {});
+  await expect(mgr.spawnFor("b", {})).rejects.toThrow(/max/i);
+  mgr.teardown("a");
+});