Phase 2: Remove dead tree-walk code from eval.sx

eval.sx: 1272 → 846 lines (-33%). sx-browser.js: 392KB → 377KB.

Deleted (superseded by CEK step handlers in cek.sx):
- eval-list: tree-walk dispatch table
- eval-call: tree-walk function dispatch
- sf-if, sf-when, sf-cond (3 variants), sf-case (2 variants)
- sf-and, sf-or, sf-let, sf-begin, sf-quote, sf-quasiquote
- sf-thread-first, sf-set!, sf-define
- ho-map, ho-filter, ho-reduce, ho-some, ho-every, ho-for-each,
  ho-map-indexed, call-fn

Kept (still called by CEK as delegates):
- sf-lambda, sf-defcomp, sf-defisland, sf-defmacro, sf-defstyle,
  sf-deftype, sf-defeffect, sf-letrec, sf-named-let
- sf-scope, sf-provide, sf-dynamic-wind
- expand-macro, qq-expand, cond-scheme?
- call-lambda, call-component, parse-keyword-args
- Strict mode, type helpers

eval-expr is now a stub overridden by CEK fixup.
All tests unchanged: JS 747/747, Full 864/870, Python 679/679.

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 md:overflow-y-auto md:overscroll-contain js-grid-viewport"
+                   :class "flex-1 md:h-full md:min-h-0 md:overflow-y-auto md:overscroll-contain overflow-x-hidden 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 md:overflow-y-auto md:overscroll-contain js-grid-viewport"
+             :class "flex-1 md:h-full md:min-h-0 md:overflow-y-auto md:overscroll-contain overflow-x-hidden js-grid-viewport"
       (when content content))))
 
 (defcomp ~shared:layout/hamburger ()