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/lib/sx_vm.ml

@@ -577,7 +577,13 @@ let jit_compile_lambda (l : lambda) globals =
     let param_syms = List (List.map (fun s -> Symbol s) l.l_params) in
     let fn_expr = List [Symbol "fn"; param_syms; l.l_body] in
     let quoted = List [Symbol "quote"; fn_expr] in
-    let result = Sx_ref.eval_expr (List [compile_fn; quoted]) (Env (make_env ())) in
+    (* Use Symbol "compile" so the CEK resolves it from the env, not
+       an embedded VmClosure value — the CEK dispatches VmClosure calls
+       differently when the value is resolved from env vs embedded in AST. *)
+    ignore compile_fn;
+    let compile_env = Sx_types.env_extend (Sx_types.make_env ()) in
+    Hashtbl.iter (fun k v -> Hashtbl.replace compile_env.bindings (Sx_types.intern k) v) globals;
+    let result = Sx_ref.eval_expr (List [Symbol "compile"; quoted]) (Env compile_env) in
     (* Closure vars are accessible via vm_closure_env (set on the VmClosure
        at line ~617). OP_GLOBAL_GET falls back to vm_closure_env when vars
        aren't in globals. No injection into the shared globals table —