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/helpers.js

@@ -19,4 +19,27 @@ async function loadPage(page, path, timeout = 15000) {
   await waitForSxReady(page);
 }
 
-module.exports = { BASE_URL, waitForSxReady, loadPage };
+/**
+ * Track console errors and uncaught exceptions on a page.
+ * Call before navigation. Use errors() to get filtered error list.
+ */
+function trackErrors(page) {
+  const raw = [];
+  page.on('pageerror', err => raw.push(err.message));
+  page.on('console', msg => {
+    if (msg.type() === 'error') raw.push(msg.text());
+  });
+  return {
+    /** Return errors, filtering transient network noise. */
+    errors() {
+      return raw.filter(e =>
+        !e.includes('Failed to fetch') &&
+        !e.includes('net::ERR') &&
+        !e.includes(' 404 ') &&
+        !e.includes('Failed to load resource')
+      );
+    }
+  };
+}
+
+module.exports = { BASE_URL, waitForSxReady, loadPage, trackErrors };