Fix stepper SSR/hydration flash: server reads cookie, cache bypass

Three changes to eliminate the stepper flash:

1. home-stepper.sx: server path reads cookie via (get-cookie) for
   step-idx initial value. Client path reads document.cookie via
   def-store. Both default to 0 when no cookie exists.

2. sx_server.ml: bypass response cache when sx-home-stepper cookie
   is present. Render on main thread (not worker) so get-cookie
   sees the parsed request cookies.

3. site-full.spec.js: flash detection test sets cookie=7 via
   Playwright context, checks SSR HTML matches hydrated state.

Test: "No flash: SSR=7 hydrated=7 (cookie=7)" — passes.
Tested on fresh stack=site server subprocess.

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

sx/sx/home-stepper.sx

@@ -18,7 +18,15 @@
                   ((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)))
+      (step-idx
+        (if
+          store
+          (get store "step-idx")
+          (let
+            ((cv (get-cookie "sx-home-stepper")))
+            (let
+              ((n (if cv (parse-number cv) 0)))
+              (signal (if (and (number? n) (>= n 0) (<= n 16)) n 0))))))
       (dom-stack-sig (signal (list)))
       (code-tokens (signal (list))))
     (letrec