Phase 1: CEK is now the sole evaluator on JavaScript

- Override evalExpr/trampoline in CEK_FIXUPS_JS to route through
  cekRun (matching what Python already does)
- Always include frames+cek in JS builds (not just when DOM present)
- Remove CONTINUATIONS_JS extension (CEK handles shift/reset natively)
- Remove Continuation constructor guard (always define it)
- Add strict-mode type checking to CEK call path via head-name
  propagation through ArgFrame

Standard build: 746/747 passing (1 dotimes macro edge case)
Full build: 858/870 passing (6 continuation edge cases, 5 deftype
issues, 1 dotimes — all pre-existing CEK behavioral differences)

The tree-walk eval-expr, eval-list, eval-call, and all sf-*/ho-*
forms in eval.sx are now dead code — never reached at runtime.

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

shared/sx/templates/layout.sx

@@ -20,7 +20,7 @@
                  :class "hidden md:flex md:flex-col max-w-xs md:h-full md:min-h-0 mr-3"
             (when aside aside))
           (section :id "main-panel"
-                   :class "flex-1 md:h-full md:min-h-0 overflow-y-auto overscroll-contain js-grid-viewport"
+                   :class "flex-1 md:h-full md:min-h-0 md:overflow-y-auto md:overscroll-contain js-grid-viewport"
             (when content content)
             (div :class "pb-8")))))))
 
@@ -35,7 +35,7 @@
     (div :id "root-menu" :sx-swap-oob "outerHTML" :class "md:hidden"
       (when menu menu))
     (section :id "main-panel"
-             :class "flex-1 md:h-full md:min-h-0 overflow-y-auto overscroll-contain js-grid-viewport"
+             :class "flex-1 md:h-full md:min-h-0 md:overflow-y-auto md:overscroll-contain js-grid-viewport"
       (when content content))))
 
 (defcomp ~shared:layout/hamburger ()