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>

hosts/ocaml/browser/compile-modules.js

@@ -37,6 +37,7 @@ const FILES = [
   'render.sx', 'core-signals.sx', 'signals.sx', 'deps.sx', 'router.sx',
   'page-helpers.sx', 'freeze.sx', 'bytecode.sx', 'compiler.sx', 'vm.sx',
   'dom.sx', 'browser.sx', 'adapter-html.sx', 'adapter-sx.sx', 'adapter-dom.sx',
+  'cssx.sx',
   'boot-helpers.sx', 'hypersx.sx', 'harness.sx', 'harness-reactive.sx',
   'harness-web.sx', 'engine.sx', 'orchestration.sx', 'boot.sx',
 ];

hosts/ocaml/browser/sx_browser.ml

@@ -647,6 +647,17 @@ let () =
   bind "provide-push!" (fun args -> match args with [n; v] -> Sx_runtime.provide_push n v | _ -> raise (Eval_error "provide-push!"));
   bind "provide-pop!" (fun args -> match args with [n] -> Sx_runtime.provide_pop n | _ -> raise (Eval_error "provide-pop!"));
 
+  (* Runtime helpers for bytecoded defcomp/defisland/defmacro forms.
+     The compiler emits GLOBAL_GET "eval-defcomp" + CALL — these must
+     exist as callable values for bytecoded .sx files that contain
+     component definitions (e.g. cssx.sx). *)
+  bind "eval-defcomp" (fun args ->
+    match args with [List (_ :: rest)] -> Sx_ref.sf_defcomp (List rest) (Env global_env) | _ -> Nil);
+  bind "eval-defisland" (fun args ->
+    match args with [List (_ :: rest)] -> Sx_ref.sf_defisland (List rest) (Env global_env) | _ -> Nil);
+  bind "eval-defmacro" (fun args ->
+    match args with [List (_ :: rest)] -> Sx_ref.sf_defmacro (List rest) (Env global_env) | _ -> Nil);
+
   (* --- Fragment / raw HTML --- *)
   bind "<>" (fun args ->
     RawHTML (String.concat "" (List.map (fun a ->
@@ -781,7 +792,13 @@ let () =
          (try Some (Sx_vm.call_closure cl args _vm_globals)
           with Eval_error msg ->
             let fn_name = match l.l_name with Some n -> n | None -> "?" in
-            Printf.eprintf "[jit] FAIL %s: %s\n%!" fn_name msg;
+            Printf.eprintf "[jit] FAIL %s: %s (bc=%d consts=%d upv=%d)\n%!"
+              fn_name msg
+              (Array.length cl.vm_code.vc_bytecode)
+              (Array.length cl.vm_code.vc_constants)
+              (Array.length cl.vm_upvalues);
+            (* Mark as failed to stop retrying *)
+            l.l_compiled <- Some (Sx_vm.jit_failed_sentinel);
             None)
        | Some _ -> None
        | None ->
@@ -796,7 +813,12 @@ let () =
              (try Some (Sx_vm.call_closure cl args _vm_globals)
               with Eval_error msg ->
                 let fn_name2 = match l.l_name with Some n -> n | None -> "?" in
-                Printf.eprintf "[jit] FAIL %s: %s\n%!" fn_name2 msg;
+                Printf.eprintf "[jit] FAIL %s: %s (bc=%d consts=%d upv=%d)\n%!"
+                  fn_name2 msg
+                  (Array.length cl.vm_code.vc_bytecode)
+                  (Array.length cl.vm_code.vc_constants)
+                  (Array.length cl.vm_upvalues);
+                l.l_compiled <- Some (Sx_vm.jit_failed_sentinel);
                 None)
            | None -> None)
          end)