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>

shared/static/wasm/sx_browser.bc.wasm.js

@@ -1792,7 +1792,7 @@
             blake2_js_for_wasm_create: blake2_js_for_wasm_create};
    }
    (globalThis))
-({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["re-9a0de245",[2]],["sx-875d5bae",[2,3]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,5]],["dune__exe__Sx_browser-def18509",[2,4,6]],["std_exit-10fb8830",[2]],["start-f808dbe1",0]],"generated":(b=>{var
+({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["re-9a0de245",[2]],["sx-624c0574",[2,3]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,5]],["dune__exe__Sx_browser-def18509",[2,4,6]],["std_exit-10fb8830",[2]],["start-f808dbe1",0]],"generated":(b=>{var
 c=b,a=b?.module?.export||b;return{"env":{"caml_ba_kind_of_typed_array":()=>{throw new
 Error("caml_ba_kind_of_typed_array not implemented")},"caml_exn_with_js_backtrace":()=>{throw new
 Error("caml_exn_with_js_backtrace not implemented")},"caml_int64_create_lo_mi_hi":()=>{throw new