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>

spec/compiler.sx

@@ -264,7 +264,7 @@
             (= name "defeffect") (emit-op em 2)
             (= name "defisland") (compile-defcomp em args scope)
             (= name "quasiquote") (compile-quasiquote em (first args) scope)
-            (= name "letrec") (compile-let em args scope tail?)
+            (= name "letrec") (compile-letrec em args scope tail?)
             ;; Default — function call
             :else
               (compile-call em head args scope tail?)))))))
@@ -387,6 +387,39 @@
       (compile-begin em body let-scope tail?))))
 
 
+(define compile-letrec
+  (fn (em args scope tail?)
+    "Compile letrec: all names visible during value compilation.
+     1. Define all local slots (initialized to nil).
+     2. Compile each value and assign — names are already in scope
+        so mutually recursive functions can reference each other."
+    (let ((bindings (first args))
+          (body (rest args))
+          (let-scope (make-scope scope)))
+      (dict-set! let-scope "next-slot" (get scope "next-slot"))
+      ;; Phase 1: define all slots (push nil for each)
+      (let ((slots (map (fn (binding)
+              (let ((name (if (= (type-of (first binding)) "symbol")
+                            (symbol-name (first binding))
+                            (first binding))))
+                (let ((slot (scope-define-local let-scope name)))
+                  (emit-op em 2)    ;; OP_NIL
+                  (emit-op em 17)   ;; OP_LOCAL_SET
+                  (emit-byte em slot)
+                  slot)))
+            bindings)))
+        ;; Phase 2: compile values and assign (all names in scope)
+        (for-each (fn (pair)
+          (let ((binding (first pair))
+                (slot (nth pair 1)))
+            (compile-expr em (nth binding 1) let-scope false)
+            (emit-op em 17)   ;; OP_LOCAL_SET
+            (emit-byte em slot)))
+          (map (fn (i) (list (nth bindings i) (nth slots i)))
+               (range 0 (len bindings)))))
+      ;; Compile body
+      (compile-begin em body let-scope tail?))))
+
 (define compile-lambda
   (fn (em args scope)
     (let ((params (first args))