Fix stepper default: show full "the joy of sx" on load

Root cause: default step-idx was 9, but the expression has 16 steps.
At step 9, only "the joy" + empty emerald span renders. Changed default
to 16 so all four words display after hydration.

Reverted mutable-list changes — (list) already creates ListRef in the
OCaml kernel, so append! works correctly with plain (list).

Added spec/tests/test-stepper.sx (7 tests) proving the split-tag +
steps-to-preview pipeline works correctly at each step boundary.

Updated Playwright stepper.spec.js with four tests:
- no raw SX visible after hydration
- default view shows all four words
- all spans inside h1
- stepping forward renders styled text

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

web/page-helpers.sx

@@ -49,7 +49,7 @@
                     "Other")))
               (when
                 (not (has-key? categories category))
-                (dict-set! categories category (mutable-list)))
+                (dict-set! categories category (list)))
               (append! (get categories category) {:doc (or (get kwargs "doc") "") :example (or (get kwargs "example") "") :tail-position (or (get kwargs "tail-position") "") :syntax (or (get kwargs "syntax") "") :name name}))))
         parsed-exprs)
       categories)))
@@ -200,7 +200,7 @@
   (fn
     ((pages-raw :as list))
     (let
-      ((pages-data (mutable-list)) (client-count 0) (server-count 0))
+      ((pages-data (list)) (client-count 0) (server-count 0))
       (for-each
         (fn
           ((page :as dict))