Fix JIT compiler, CSSX browser support, double-fetch, SPA layout

JIT compiler:
- Fix jit_compile_lambda: resolve `compile` via symbol lookup in env
  instead of embedding VmClosure in AST (CEK dispatches differently)
- Register eval-defcomp/eval-defisland/eval-defmacro runtime helpers
  in browser kernel for bytecoded defcomp forms
- Disable broken .sxbc.json path (missing arity in nested code blocks),
  use .sxbc text format only
- Mark JIT-failed closures as sentinel to stop retrying

CSSX in browser:
- Add cssx.sx symlink + cssx.sxbc to browser web stack
- Add flush-cssx! to orchestration.sx post-swap for SPA nav
- Add cssx.sx to compile-modules.js and mcp_tree.ml bytecode lists

SPA navigation:
- Fix double-fetch: check e.defaultPrevented in click delegation
  (bind-event already handled the click)
- Fix layout destruction: change nav links from outerHTML to innerHTML
  swap (outerHTML destroyed #main-panel when response lacked it)
- Guard JS popstate handler when SX engine is booted
- Rename sx-platform.js → sx-platform-2.js to bust immutable cache

Playwright tests:
- Add trackErrors() helper to all test specs
- Add SPA DOM comparison test (SPA nav vs fresh load)
- Add single-fetch + no-duplicate-elements test
- Improve MCP tool output: show failure details and error messages

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

tests/playwright/reactive-nav.spec.js

@@ -1,8 +1,11 @@
 // @ts-check
 const { test, expect } = require('playwright/test');
-const { BASE_URL, waitForSxReady } = require('./helpers');
+const { BASE_URL, waitForSxReady, trackErrors } = require('./helpers');
 
 test.describe('Reactive Island Navigation', () => {
+  let t;
+  test.beforeEach(({ page }) => { t = trackErrors(page); });
+  test.afterEach(() => { expect(t.errors()).toEqual([]); });
 
   test('counter island works on direct load', async ({ page }) => {
     await page.goto(BASE_URL + '/sx/(geography.(reactive.(examples.counter)))', { waitUntil: 'domcontentloaded' });