stk: never shadow a paused command with a new menu selection

The stuck-card pattern: after a Back TR the card re-issues the parent menu
as a new SELECT ITEM (91XX -> FETCH, paused, awaiting TR), but the UI's
back/timeout branch ignored that response and showed the cached top menu;
the next item click then sent ENVELOPE(Menu Selection) while the card was
waiting for the TR. The card answers such an ENVELOPE with 9000 (not the
usual 91XX), and menu-select cleared the pending command without a TR,
leaving an unfinished proactive session until reset/equip.

- server: _finish_pending_menu() answers a paused command with a cancel TR
  (0x10) before /api/menu-select sends its ENVELOPE and drains a 91XX
  follow-up, so a new selection can never shadow an unanswered FETCH
- frontend: stkMenuRespond('back'|'timeout') renders the follow-up command
  from the server response (SELECT ITEM / DISPLAY TEXT) and shows the
  cached top menu only when the TR answer carries no command (9000)
- tests: finish-pending cancel TR + chain drain (Python); stkMenuRespond
  back/timeout/cancel/ok rendering (frontend); help/AGENTS updated;
  SW cache v107 -> v108.

frontend/tests/stk_menu.test.js

@@ -0,0 +1,85 @@
+const { test } = require('node:test');
+const assert = require('node:assert');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const html = fs.readFileSync(path.join(__dirname, '..', 'index.html'), 'utf8');
+
+function extractFunc(src, name, asyncFn) {
+	const re = new RegExp('function\\s+' + name + '\\s*\\([^)]*\\)\\s*\\{');
+	const m = re.exec(src);
+	if (!m) throw new Error('function ' + name + ' not found');
+	let i = m.index + m[0].length - 1;
+	let depth = 0;
+	for (; i < src.length; i++) {
+		if (src[i] === '{') depth++;
+		else if (src[i] === '}') {
+			depth--;
+			if (depth === 0) break;
+		}
+	}
+	return (asyncFn ? 'async ' : '') + src.slice(m.index, i + 1);
+}
+
+let code = extractFunc(html, 'stkMenuRespond', true) + '\n';
+code += 'globalThis.esc = s => s;\n';
+eval(code);
+
+function setup(response) {
+	const calls = { handled: null, rendered: 0 };
+	globalThis.pysimFetch = async () => response;
+	globalThis.stkMenuHandleResponse = d => { calls.handled = d; };
+	globalThis.stkMenuRenderItems = () => { calls.rendered++; };
+	const btns = { classList: { add: () => {} } };
+	const back = { style: {} };
+	globalThis.document = {
+		getElementById: id => (id === 'stk-menu-buttons' ? btns : id === 'stk-back-btn' ? back : { innerHTML: '' }),
+	};
+	return calls;
+}
+
+test('back with a fetched SELECT ITEM continues the card dialogue', async () => {
+	const data = { type: 'select_item', items: [{ id: 1, text: 'Info' }] };
+	const calls = setup(data);
+	await stkMenuRespond('back');
+	assert.strictEqual(calls.handled, data);
+	assert.strictEqual(calls.rendered, 0);
+});
+
+test('back with a fetched DISPLAY TEXT shows it', async () => {
+	const data = { type: 'display_text', text: 'hello' };
+	const calls = setup(data);
+	await stkMenuRespond('back');
+	assert.strictEqual(calls.handled, data);
+	assert.strictEqual(calls.rendered, 0);
+});
+
+test('timeout with a fetched SELECT ITEM continues the card dialogue', async () => {
+	const data = { type: 'select_item', items: [] };
+	const calls = setup(data);
+	await stkMenuRespond('timeout');
+	assert.strictEqual(calls.handled, data);
+	assert.strictEqual(calls.rendered, 0);
+});
+
+test('back answered with SW 9000 falls back to the cached top menu', async () => {
+	const calls = setup({ type: 'done', sw: '9000' });
+	await stkMenuRespond('back');
+	assert.strictEqual(calls.handled, null);
+	assert.strictEqual(calls.rendered, 1);
+});
+
+test('cancel falls back to the cached top menu', async () => {
+	const calls = setup({ sw: '9000' });
+	await stkMenuRespond('cancel');
+	assert.strictEqual(calls.handled, null);
+	assert.strictEqual(calls.rendered, 1);
+});
+
+test('ok navigates with the server response', async () => {
+	const data = { type: 'select_item', items: [{ id: 1, text: 'x' }] };
+	const calls = setup(data);
+	await stkMenuRespond('ok');
+	assert.strictEqual(calls.handled, data);
+	assert.strictEqual(calls.rendered, 0);
+});