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>

shared/static/wasm/sx/cssx.sx

@@ -0,0 +1 @@
+../../../../shared/sx/templates/cssx.sx

shared/static/wasm/sx/orchestration.sx

@@ -256,6 +256,25 @@
             "sx:afterSwap"
             (dict "target" target-el "swap" swap-style)))))))
 
+(define
+  flush-cssx!
+  :effects (mutation io)
+  (fn
+    ()
+    (let
+      ((rules (collected "cssx")))
+      (clear-collected! "cssx")
+      (when
+        (not (empty? rules))
+        (let
+          ((style (dom-query "#sx-css")))
+          (when
+            style
+            (dom-set-prop
+              style
+              "textContent"
+              (str (dom-get-prop style "textContent") (join "" rules)))))))))
+
 (define
   handle-sx-response
   :effects (mutation io)
@@ -508,7 +527,8 @@
     (sx-hydrate root)
     (sx-hydrate-islands root)
     (run-post-render-hooks)
-    (process-elements root)))
+    (process-elements root)
+    (flush-cssx!)))
 
 (define
   process-settle-hooks