Fix stepper flash: SSR and client both start at step 0

Previously SSR rendered at step 16 (hardcoded) but client initialized
from cookie, causing a flash from 16 to the cookie value on return visits.

Fix: Both SSR and client default to step 0. The def-store initializer
reads the cookie for the client's initial value. Return visits show
a progressive fill (0 → cookie value) instead of a jarring state jump.

- step-idx default: (signal 0) in both SSR and client paths
- def-store: reads sx-home-stepper cookie for initial value, defaults to 0
- Removed redundant post-hydration cookie reset block

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

sx/sx/home-stepper.sx

@@ -5,8 +5,20 @@
     ((source "(div (~tw :tokens \"text-center\")\n  (h1 (~tw :tokens \"text-3xl font-bold mb-2\")\n    (span (~tw :tokens \"text-rose-500\") \"the \")\n    (span (~tw :tokens \"text-amber-500\") \"joy \")\n    (span (~tw :tokens \"text-emerald-500\") \"of \")\n    (span (~tw :tokens \"text-violet-600 text-4xl\") \"sx\")))")
       (steps (signal (list)))
       (store
-        (if (client?) (def-store "home-stepper" (fn () {:step-idx (signal 16)})) nil))
-      (step-idx (if store (get store "step-idx") (signal 16)))
+        (if
+          (client?)
+          (def-store
+            "home-stepper"
+            (fn
+              ()
+              (let
+                ((raw (host-get (host-global "document") "cookie"))
+                  (prefix "sx-home-stepper="))
+                (let
+                  ((val (when (and (string? raw) (contains? raw prefix)) (let ((start (+ (index-of raw prefix) (len prefix)))) (let ((rest (slice raw start)) (end-pos (index-of (slice raw start) ";"))) (if (> end-pos -1) (slice rest 0 end-pos) rest))))))
+                  (let ((n (if val (parse-number val) 0))) {:step-idx (signal (if (and (number? n) (>= n 0) (<= n 16)) n 0))})))))
+          nil))
+      (step-idx (if store (get store "step-idx") (signal 0)))
       (dom-stack-sig (signal (list)))
       (code-tokens (signal (list))))
     (letrec
@@ -310,15 +322,7 @@
                     "sx-home-stepper="
                     target
                     ";path=/;max-age=31536000;SameSite=Lax")))))))
-      (let
-        ((saved (if (client?) (let ((raw (host-get (dom-document) "cookie")) (prefix "sx-home-stepper=")) (when (and (string? raw) (contains? raw prefix)) (let ((start (+ (index-of raw prefix) (len prefix)))) (let ((rest (slice raw start)) (end-pos (index-of rest ";"))) (if (> end-pos -1) (slice rest 0 end-pos) rest))))) (get-cookie "sx-home-stepper"))))
-        (when
-          (string? saved)
-          (let
-            ((n (parse-number saved)))
-            (when
-              (and (number? n) (>= n 0) (<= n 16))
-              (reset! step-idx n)))))
+      nil
       (let
         ((parsed (sx-parse source)))
         (when