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>

shared/static/scripts/sx-browser.js

@@ -14,7 +14,7 @@
   // =========================================================================
 
   var NIL = Object.freeze({ _nil: true, toString: function() { return "nil"; } });
-  var SX_VERSION = "2026-03-24T20:05:12Z";
+  var SX_VERSION = "2026-03-24T22:31:01Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }

shared/sx/ocaml_bridge.py

@@ -228,9 +228,12 @@ class OcamlBridge:
         if not cookies:
             return
         # Build SX dict: {:name1 "val1" :name2 "val2"}
+        # Cookie values may be URL-encoded (client set-cookie uses
+        # encodeURIComponent) — decode before sending to kernel.
+        from urllib.parse import unquote
         pairs = []
         for k, v in cookies.items():
-            pairs.append(f':{k} "{_escape(str(v))}"')
+            pairs.append(f':{k} "{_escape(unquote(str(v)))}"')
         if pairs:
             cmd = f'(set-request-cookies {{{" ".join(pairs)}}})'
             try: