Fix: local bindings now shadow HTML tag special forms in browser evaluator

Root cause: sx_browser.ml registered all HTML tags (a, b, i, p, s, u, g, etc.)
as custom special forms. The evaluator's step_eval_list checked custom special
forms BEFORE checking local env bindings. So (let ((a (fn () 42))) (a))
matched the HTML tag <a> instead of calling the local function a.

Fix: skip custom special forms AND render-check when the symbol is bound in
the local env. Added (not (env-has? env name)) guard to both checks in
step-eval-list (spec/evaluator.sx and transpiled sx_ref.ml).

This was the root cause of "[sx] resume: Not callable: nil" — after hs-wait
resumed, calling letrec-bound functions like wait-boot (which is not an HTML
tag) worked, but any function whose name collided with an HTML tag failed.

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

spec/evaluator.sx

@@ -1820,7 +1820,9 @@
               ("define-syntax" (step-sf-define args env kont))
               (_
                 (cond
-                  (has-key? *custom-special-forms* name)
+                  (and
+                    (has-key? *custom-special-forms* name)
+                    (not (env-has? env name)))
                   (make-cek-value
                     ((get *custom-special-forms* name) args env)
                     env
@@ -1829,7 +1831,10 @@
                   (let
                     ((mac (env-get env name)))
                     (make-cek-state (expand-macro mac args env) env kont))
-                  (and *render-check* (*render-check* expr env))
+                  (and
+                    *render-check*
+                    (not (env-has? env name))
+                    (*render-check* expr env))
                   (make-cek-value (*render-fn* expr env) env kont)
                   :else (step-eval-call head args env kont)))))
           (step-eval-call head args env kont))))))