tui: add state-file path resolver (Windows + XDG)

Resolves the location of the cross-process TUI state file:
- Windows: %LOCALAPPDATA%/codesys-mcp/tui-state.json
- POSIX: $XDG_STATE_HOME/codesys-mcp/tui-state.json, falling back to
  $HOME/.local/state/codesys-mcp/tui-state.json

Uses path.win32 / path.posix explicitly so the POSIX branches still
produce POSIX-flavored paths when this runs on a Windows host (test
machine), keeping the unit tests host-independent.

src/tui/shared/state-paths.ts

@@ -0,0 +1,19 @@
+import * as os from 'os';
+import * as path from 'path';
+
+const APP_DIR = 'codesys-mcp';
+const FILE_NAME = 'tui-state.json';
+
+export function stateFilePath(): string {
+  if (process.platform === 'win32') {
+    const localAppData = process.env.LOCALAPPDATA;
+    if (!localAppData) {
+      throw new Error('LOCALAPPDATA is not set; cannot resolve TUI state file path');
+    }
+    return path.win32.join(localAppData, APP_DIR, FILE_NAME);
+  }
+  const xdg = process.env.XDG_STATE_HOME;
+  const home = process.env.HOME ?? os.homedir();
+  const base = xdg ?? path.posix.join(home, '.local', 'state');
+  return path.posix.join(base, APP_DIR, FILE_NAME);
+}

tests/tui/state-paths.test.ts

@@ -0,0 +1,51 @@
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import * as path from 'path';
+import { stateFilePath } from '../../src/tui/shared/state-paths.ts';
+
+const ORIG_PLATFORM = Object.getOwnPropertyDescriptor(process, 'platform')!;
+const ORIG_ENV = { ...process.env };
+
+function setPlatform(p: NodeJS.Platform) {
+  Object.defineProperty(process, 'platform', { value: p, configurable: true });
+}
+
+beforeEach(() => {
+  process.env = { ...ORIG_ENV };
+});
+
+afterEach(() => {
+  Object.defineProperty(process, 'platform', ORIG_PLATFORM);
+  process.env = { ...ORIG_ENV };
+});
+
+describe('stateFilePath', () => {
+  it('uses %LOCALAPPDATA%/codesys-mcp/tui-state.json on Windows', () => {
+    setPlatform('win32');
+    process.env.LOCALAPPDATA = 'C:\\\\Users\\\\u\\\\AppData\\\\Local';
+    const p = stateFilePath();
+    expect(p).toBe(
+      path.join('C:\\\\Users\\\\u\\\\AppData\\\\Local', 'codesys-mcp', 'tui-state.json')
+    );
+  });
+
+  it('uses $XDG_STATE_HOME/codesys-mcp/tui-state.json when set', () => {
+    setPlatform('linux');
+    process.env.XDG_STATE_HOME = '/tmp/xdg-state';
+    const p = stateFilePath();
+    expect(p).toBe('/tmp/xdg-state/codesys-mcp/tui-state.json');
+  });
+
+  it('falls back to ~/.local/state on Linux without XDG_STATE_HOME', () => {
+    setPlatform('linux');
+    delete process.env.XDG_STATE_HOME;
+    process.env.HOME = '/home/u';
+    const p = stateFilePath();
+    expect(p).toBe('/home/u/.local/state/codesys-mcp/tui-state.json');
+  });
+
+  it('throws on Windows when LOCALAPPDATA is unset', () => {
+    setPlatform('win32');
+    delete process.env.LOCALAPPDATA;
+    expect(() => stateFilePath()).toThrow(/LOCALAPPDATA/);
+  });
+});