fix: persist state under plugin-state/ (runner de-privilege)

The managed runner is de-privileged on Windows now (runs as LocalService),
and %ProgramData%\punktfunk is locked read-only to it — so writing config/
cache straight under the config dir fails EPERM. Move the plugin's state to
<config_dir>/plugin-state/playnite, which `punktfunk-host plugins enable`
grants the runner write on. On Linux the runner owns the config dir, so the
path is writable there too. First release, so no migration needed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/state.test.ts

@@ -0,0 +1,36 @@
+// State persistence lands under `<config_dir>/plugin-state/playnite` — the one dir the
+// de-privileged Windows runner (LocalService) may write. A regression here (writing straight under
+// the config dir) would EPERM under the runner and lose the operator's config.
+import { afterEach, beforeEach, describe, expect, test } from "bun:test";
+import * as fs from "node:fs";
+import * as os from "node:os";
+import * as path from "node:path";
+import { loadConfig, saveRawConfig } from "../src/state.js";
+
+let root: string;
+let saved: string | undefined;
+
+beforeEach(() => {
+	saved = process.env.PUNKTFUNK_CONFIG_DIR;
+	root = fs.mkdtempSync(path.join(os.tmpdir(), "pn-state-"));
+	process.env.PUNKTFUNK_CONFIG_DIR = root;
+});
+afterEach(() => {
+	if (saved === undefined) delete process.env.PUNKTFUNK_CONFIG_DIR;
+	else process.env.PUNKTFUNK_CONFIG_DIR = saved;
+	fs.rmSync(root, { recursive: true, force: true });
+});
+
+describe("state location", () => {
+	test("persists under plugin-state/playnite and round-trips", () => {
+		saveRawConfig({ playniteDir: "/games/playnite" });
+		expect(
+			fs.existsSync(path.join(root, "plugin-state", "playnite", "config.json")),
+		).toBe(true);
+		// Not written straight under the config dir (the LocalService-unwritable location).
+		expect(fs.existsSync(path.join(root, "playnite", "config.json"))).toBe(
+			false,
+		);
+		expect(loadConfig().playniteDir).toBe("/games/playnite");
+	});
+});