Fix isomorphic SSR: revert inline opcodes, add named let compilation, fix cookie decode

Three bugs broke island SSR rendering of the home stepper widget:

1. Inline VM opcodes (OP_ADD..OP_DEC) broke JIT-compiled functions.
   The compiler emitted single-byte opcodes for first/rest/len/= etc.
   that produced wrong results in complex recursive code (sx-parse
   returned nil, split-tag produced 1 step instead of 16). Reverted
   compiler to use CALL_PRIM for all primitives. VM opcode handlers
   kept for future use.

2. Named let (let loop ((x init)) body) had no compiler support —
   silently produced broken bytecode. Added desugaring to letrec.

3. URL-encoded cookie values not decoded server-side. Client set-cookie
   uses encodeURIComponent but Werkzeug doesn't decode cookie values.
   Added unquote() in bridge cookie injection.

Also: call-lambda used eval_expr which copies Dict values (signals),
breaking mutations through aser lambda calls. Switched to cek_call.

Also: stepper preview now includes ~cssx/tw spreads for SSR styling.

Tests: 1317 JS, 1114 OCaml, 26 integration (2 pre-existing failures)

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

spec/tests/vm-inline.sx

@@ -83,3 +83,35 @@
 (test "= with booleans" (= (= true true) true))
 (test "= with keywords" (= (= :foo :foo) true))
 (test "not with list" (= (not (list 1)) false))
+
+;; --------------------------------------------------------------------------
+;; Recursive mutation — VM closure capture must preserve mutable state
+;; --------------------------------------------------------------------------
+;;
+;; Regression: recursive functions that append! to a shared mutable list
+;; lost mutations after the first call under JIT.  The VM closure capture
+;; was copying the list value instead of sharing the mutable reference.
+
+(test "recursive append! to shared list"
+  (let ((walk (fn (items result)
+          (when (not (empty? items))
+            (append! result (first items))
+            (walk (rest items) result)))))
+    (let ((result (list)))
+      (walk (list "a" "b" "c") result)
+      (= (len result) 3))))
+
+(test "recursive tree walk with append!"
+  (let ((walk (fn (expr result)
+          (cond
+            (not (list? expr))
+              (append! result "leaf")
+            (empty? expr) nil
+            :else
+              (do (append! result "open")
+                  (for-each (fn (c) (walk c result)) (rest expr))
+                  (append! result "close"))))))
+    (let ((tree (first (sx-parse "(div \"a\" (span \"b\") \"c\")")))
+          (result (list)))
+      (walk tree result)
+      (= (len result) 7))))