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/ref/primitives.sx

@@ -121,7 +121,7 @@
 (define-primitive "="
   :params (a b)
   :returns "boolean"
-  :doc "Equality (value equality, not identity).")
+  :doc "Deep structural equality. Alias for equal?.")
 
 (define-primitive "!="
   :params (a b)
@@ -129,6 +129,27 @@
   :doc "Inequality."
   :body (not (= a b)))
 
+(define-primitive "eq?"
+  :params (a b)
+  :returns "boolean"
+  :doc "Identity equality. True only if a and b are the exact same object.
+        For immutable atoms (numbers, strings, booleans, nil) this may or
+        may not match — use eqv? for reliable atom comparison.")
+
+(define-primitive "eqv?"
+  :params (a b)
+  :returns "boolean"
+  :doc "Equivalent value for atoms, identity for compound objects.
+        Returns true for identical objects (eq?), and also for numbers,
+        strings, booleans, and nil with the same value. For lists, dicts,
+        lambdas, and components, only true if same identity.")
+
+(define-primitive "equal?"
+  :params (a b)
+  :returns "boolean"
+  :doc "Deep structural equality. Recursively compares lists and dicts.
+        Same semantics as = but explicit Scheme name.")
+
 (define-primitive "<"
   :params (a b)
   :returns "boolean"