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/bootstrap_js.py

@@ -165,6 +165,12 @@ class JSEmitter:
             "sf-and": "sfAnd",
             "sf-or": "sfOr",
             "sf-let": "sfLet",
+            "sf-named-let": "sfNamedLet",
+            "sf-letrec": "sfLetrec",
+            "sf-dynamic-wind": "sfDynamicWind",
+            "push-wind!": "pushWind",
+            "pop-wind!": "popWind",
+            "call-thunk": "callThunk",
             "sf-lambda": "sfLambda",
             "sf-define": "sfDefine",
             "sf-defcomp": "sfDefcomp",