Fix navigation: deep URL routing, back button, render timeout

- request-handler.sx: replace all dots (not just `.(`) and auto-quote
  undefined symbols as strings so 3-level URLs like
  /sx/(geography.(reactive.(examples.counter))) resolve correctly
- sx-platform.js: register popstate handler (was missing from manual
  boot sequence) and fetch full HTML for back/forward navigation
- sx_ref.ml: add CEK step limit (10M steps) checked every 4096 steps
  so runaway renders return 500 instead of blocking the worker forever
- Rename test-runner.sx → runner-placeholder.sx to avoid `test-` skip
- Playwright config: pin testDir, single worker, ignore worktrees

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/playwright/playwright.config.js

@@ -2,9 +2,11 @@
 const { defineConfig } = require('playwright/test');
 
 module.exports = defineConfig({
-  testDir: '.',
+  testDir: __dirname,
+  testMatch: '*.spec.js',
   timeout: 60000,
   retries: 0,
+  workers: 1,
   use: {
     baseURL: process.env.SX_TEST_URL || 'http://localhost:8013',
     headless: true,