Fix stepper: mutable-list for append! in split-tag and steps-to-preview

The stepper's split-tag and steps-to-preview used (list) with append!,
but in the WASM kernel (list) creates immutable List values — append!
returns a new list without mutating, so children accumulate nowhere.

Changed all accumulator initializations to (mutable-list):
- split-tag: cch, cat, spreads
- steps-to-preview: bc-loop inner children, initial call
- result and tokens lists in the parsing setup

Also includes WASM rebuild with append! primitive and &rest fixes.

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

shared/static/wasm/sx/core-signals.sx

@@ -1,23 +1,17 @@
-;; Create raw signal dict with value, subs, deps fields
 (define
   make-signal
   (fn
     (value)
     (dict "__signal" true "value" value "subscribers" (list) "deps" (list))))
 
-;; Type predicate for signals
 (define signal? (fn (x) (and (dict? x) (has-key? x "__signal"))))
 
-;; Read current value from signal
 (define signal-value (fn (s) (get s "value")))
 
-;; Write value to signal (no notification)
 (define signal-set-value! (fn (s v) (dict-set! s "value" v)))
 
-;; List of subscriber functions
 (define signal-subscribers (fn (s) (get s "subscribers")))
 
-;; Add a subscriber function
 (define
   signal-add-sub!
   (fn
@@ -26,7 +20,6 @@
       (not (contains? (get s "subscribers") f))
       (dict-set! s "subscribers" (append (get s "subscribers") (list f))))))
 
-;; Remove a subscriber function
 (define
   signal-remove-sub!
   (fn
@@ -36,19 +29,15 @@
       "subscribers"
       (filter (fn (sub) (not (identical? sub f))) (get s "subscribers")))))
 
-;; List of upstream signal dependencies
 (define signal-deps (fn (s) (get s "deps")))
 
-;; Set upstream dependencies
 (define signal-set-deps! (fn (s deps) (dict-set! s "deps" deps)))
 
-;; Create a reactive signal (user-facing constructor)
 (define
   signal
   :effects ()
   (fn ((initial-value :as any)) (make-signal initial-value)))
 
-;; Dereference a signal, returning its current value
 (define
   deref
   :effects ()
@@ -69,7 +58,6 @@
               (signal-add-sub! s notify-fn))))
         (signal-value s)))))
 
-;; Set signal to new value and notify subscribers
 (define
   reset!
   :effects (mutation)
@@ -84,12 +72,11 @@
           (signal-set-value! s value)
           (notify-subscribers s))))))
 
-;; Apply function to current value and reset
 (define
   swap!
   :effects (mutation)
   (fn
-    ((s :as signal) (f :as lambda) &rest args)
+    ((s :as signal) (f :as callable) &rest args)
     (when
       (signal? s)
       (let
@@ -100,7 +87,6 @@
           (signal-set-value! s new-val)
           (notify-subscribers s))))))
 
-;; Create a derived signal that auto-updates from dependencies
 (define
   computed
   :effects (mutation)
@@ -114,7 +100,6 @@
         (register-in-scope (fn () (dispose-computed s)))
         s))))
 
-;; Create a side-effect that runs when dependencies change
 (define
   effect
   :effects (mutation)
@@ -130,13 +115,10 @@
           (register-in-scope dispose-fn)
           dispose-fn)))))
 
-;; Nesting counter for batched updates
 (define *batch-depth* 0)
 
-;; Queued notifications during batch
 (define *batch-queue* (list))
 
-;; Batch multiple signal updates, notify once at end
 (define
   batch
   :effects (mutation)
@@ -166,7 +148,6 @@
             queue)
           (for-each (fn ((sub :as lambda)) (sub)) pending))))))
 
-;; Notify all subscribers of a signal change
 (define
   notify-subscribers
   :effects (mutation)
@@ -177,7 +158,6 @@
       (when (not (contains? *batch-queue* s)) (append! *batch-queue* s))
       (flush-subscribers s))))
 
-;; Process queued subscriber notifications
 (define
   flush-subscribers
   :effects (mutation)
@@ -185,7 +165,6 @@
     ((s :as dict))
     (for-each (fn (sub) (cek-call sub nil)) (signal-subscribers s))))
 
-;; Tear down a computed signal, remove from deps
 (define
   dispose-computed
   :effects (mutation)
@@ -198,7 +177,6 @@
         (signal-deps s))
       (signal-set-deps! s (list)))))
 
-;; Evaluate body in an island disposal scope
 (define
   with-island-scope
   :effects (mutation)
@@ -207,7 +185,6 @@
     (scope-push! "sx-island-scope" scope-fn)
     (let ((result (body-fn))) (scope-pop! "sx-island-scope") result)))
 
-;; Register a disposable in the current island scope
 (define
   register-in-scope
   :effects (mutation)