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>

sx/sx/home-stepper.sx

@@ -116,10 +116,10 @@
                                 (dict-set! pos "i" (+ (get pos "i") 1))
                                 (let ((tag (get step "tag"))
                                       (attrs (or (get step "attrs") (list)))
+                                      (spreads (or (get step "spreads") (list)))
                                       (inner (build-children)))
-                                  ;; Skip spreads (~cssx/tw) — just structure + text
                                   (append! children
-                                    (concat (list (make-symbol tag)) attrs inner)))
+                                    (concat (list (make-symbol tag)) spreads attrs inner)))
                                 (loop))
                             (= stype "close")
                               (do (dict-set! pos "i" (+ (get pos "i") 1))