Add Scheme forms: named let, letrec, dynamic-wind, three-tier equality

Spec (eval.sx, primitives.sx):
- Named let: (let loop ((i 0)) body) — self-recursive lambda with TCO
- letrec: mutually recursive local bindings with closure patching
- dynamic-wind: entry/exit guards with wind stack for future continuations
- eq?/eqv?/equal?: identity, atom-value, and deep structural equality

Implementation (evaluator.py, async_eval.py, primitives.py):
- Both sync and async evaluators implement all four forms
- 33 new tests covering all forms including TCO at 10k depth

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/primitives.py

@@ -132,6 +132,27 @@ def prim_eq(a: Any, b: Any) -> bool:
 def prim_neq(a: Any, b: Any) -> bool:
     return a != b
 
+@register_primitive("eq?")
+def prim_eq_identity(a: Any, b: Any) -> bool:
+    """Identity equality — true only if a and b are the same object."""
+    return a is b
+
+@register_primitive("eqv?")
+def prim_eqv(a: Any, b: Any) -> bool:
+    """Equivalent: identity for compound types, value for atoms."""
+    if a is b:
+        return True
+    if isinstance(a, (int, float, str, bool)) and isinstance(b, type(a)):
+        return a == b
+    if (a is None or a is NIL) and (b is None or b is NIL):
+        return True
+    return False
+
+@register_primitive("equal?")
+def prim_equal(a: Any, b: Any) -> bool:
+    """Deep structural equality (same as =)."""
+    return a == b
+
 @register_primitive("<")
 def prim_lt(a: Any, b: Any) -> bool:
     return a < b