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>

web/tests/test-aser.sx

@@ -432,3 +432,37 @@
       (render-sx
         "(do (defcomp ~page (&key x) (div x))
              (case \"miss\" \"hit\" (~page :x \"found\") :else \"index\"))"))))
+
+
+;; --------------------------------------------------------------------------
+;; Dict mutation through lambda calls in aser body
+;; --------------------------------------------------------------------------
+;;
+;; Regression: aser's :else branch used call-lambda which re-evaluated
+;; args through eval_expr.  The CEK evaluator copies Dict values during
+;; evaluation (treating them as dict literals), so mutations inside the
+;; lambda operated on a copy, not the original.  This broke signal
+;; reset!/swap! in island SSR where aser processes multi-body let forms.
+
+(defsuite "aser-dict-mutation"
+  (deftest "lambda mutating dict arg in multi-body let"
+    (assert-equal "99"
+      (render-sx
+        "(let ((mutate! (fn (d k v) (dict-set! d k v)))
+               (d (dict \"x\" 1)))
+           (mutate! d \"x\" 99)
+           (get d \"x\"))")))
+
+  (deftest "signal reset! in multi-body let"
+    (assert-equal "99"
+      (render-sx
+        "(let ((s (signal 42)))
+           (reset! s 99)
+           (deref s))")))
+
+  (deftest "signal reset! then len of deref in multi-body let"
+    (assert-equal "3"
+      (render-sx
+        "(let ((s (signal (list))))
+           (reset! s (list 1 2 3))
+           (len (deref s)))"))))