Compiler: proper letrec support (mutual recursion)

The compiler was treating letrec as let — binding values sequentially.
This meant mutually recursive functions (like sx-parse's read-list
calling read-expr and vice versa) couldn't reference each other.

compile-letrec uses two phases:
1. Define all local slots initialized to nil
2. Compile and assign values — all names already in scope

This fixes sx-parse producing wrong ASTs (nested instead of sibling
lists) when JIT-compiled, which caused the stepper's step count to
be 2 instead of 16.

Also: skip JIT for lambdas with closure bindings (inner functions
like read-list-loop) — the closure merging into vm_env_ref produces
incorrect variable resolution.

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 = "2026-03-23T18:18:45Z";
+  var SX_VERSION = "2026-03-23T18:52:19Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }