Split env-bind! from env-set!: fix lexical scoping and closures

Two fundamental environment bugs fixed:

1. env-set! was used for both binding creation (let, define, params)
   and mutation (set!). Binding creation must NOT walk the scope chain
   — it should set on the immediate env. Only set! should walk.

   Fix: introduce env-bind! for all binding creation. env-set! now
   exclusively means "mutate existing binding, walk scope chain".
   Changed across spec (eval.sx, cek.sx, render.sx) and all web
   adapters (dom, html, sx, async, boot, orchestration, forms).

2. makeLambda/makeComponent/makeMacro/makeIsland used merge(env) to
   flatten the closure into a plain object, destroying the prototype
   chain. This meant set! inside closures couldn't reach the original
   binding — it modified a snapshot copy instead.

   Fix: store env directly as closure (no merge). The prototype chain
   is preserved, so set! walks up to the original scope.

Tests: 499/516 passing (96.7%), up from 485/516.
Fixed: define self-reference, let scope isolation, set! through
closures, counter-via-closure pattern, recursive functions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/python/platform.py

@@ -498,10 +498,23 @@ def env_get(env, name):
     return env.get(name, NIL)
 
 
-def env_set(env, name, val):
+def env_bind(env, name, val):
+    """Create/overwrite binding on THIS env only (let, define, param binding)."""
     env[name] = val
 
 
+def env_set(env, name, val):
+    """Mutate existing binding, walking scope chain (set!)."""
+    if hasattr(env, 'set'):
+        try:
+            env.set(name, val)
+        except KeyError:
+            # Not found anywhere — bind on immediate env
+            env[name] = val
+    else:
+        env[name] = val
+
+
 def env_extend(env):
     return _ensure_env(env).extend()
 

hosts/python/transpiler.sx

@@ -107,6 +107,7 @@
   "get-primitive" "get_primitive"
   "env-has?" "env_has"
   "env-get" "env_get"
+  "env-bind!" "env_bind"
   "env-set!" "env_set"
   "env-extend" "env_extend"
   "env-merge" "env_merge"
@@ -524,11 +525,16 @@
                    ", " (py-expr-with-cells (nth args 1) cell-vars)
                    ", " (py-expr-with-cells (nth args 2) cell-vars) ")")
 
-            (= op "env-set!")
+            (= op "env-bind!")
               (str "_sx_dict_set(" (py-expr-with-cells (nth args 0) cell-vars)
                    ", " (py-expr-with-cells (nth args 1) cell-vars)
                    ", " (py-expr-with-cells (nth args 2) cell-vars) ")")
 
+            (= op "env-set!")
+              (str "env_set(" (py-expr-with-cells (nth args 0) cell-vars)
+                   ", " (py-expr-with-cells (nth args 1) cell-vars)
+                   ", " (py-expr-with-cells (nth args 2) cell-vars) ")")
+
             (= op "set-lambda-name!")
               (str "_sx_set_attr(" (py-expr-with-cells (nth args 0) cell-vars)
                    ", 'name', " (py-expr-with-cells (nth args 1) cell-vars) ")")
@@ -901,10 +907,14 @@
             (= name "append!")
               (str pad (py-expr-with-cells (nth expr 1) cell-vars)
                    ".append(" (py-expr-with-cells (nth expr 2) cell-vars) ")")
-            (= name "env-set!")
+            (= name "env-bind!")
               (str pad (py-expr-with-cells (nth expr 1) cell-vars)
                    "[" (py-expr-with-cells (nth expr 2) cell-vars)
                    "] = " (py-expr-with-cells (nth expr 3) cell-vars))
+            (= name "env-set!")
+              (str pad "env_set(" (py-expr-with-cells (nth expr 1) cell-vars)
+                   ", " (py-expr-with-cells (nth expr 2) cell-vars)
+                   ", " (py-expr-with-cells (nth expr 3) cell-vars) ")")
             (= name "set-lambda-name!")
               (str pad (py-expr-with-cells (nth expr 1) cell-vars)
                    ".name = " (py-expr-with-cells (nth expr 2) cell-vars))
@@ -1098,10 +1108,14 @@
                 (append! lines (str pad (py-expr-with-cells (nth expr 1) cell-vars)
                                     "[" (py-expr-with-cells (nth expr 2) cell-vars)
                                     "] = " (py-expr-with-cells (nth expr 3) cell-vars)))
-              (= name "env-set!")
+              (= name "env-bind!")
                 (append! lines (str pad (py-expr-with-cells (nth expr 1) cell-vars)
                                     "[" (py-expr-with-cells (nth expr 2) cell-vars)
                                     "] = " (py-expr-with-cells (nth expr 3) cell-vars)))
+              (= name "env-set!")
+                (append! lines (str pad "env_set(" (py-expr-with-cells (nth expr 1) cell-vars)
+                                    ", " (py-expr-with-cells (nth expr 2) cell-vars)
+                                    ", " (py-expr-with-cells (nth expr 3) cell-vars) ")"))
               :else
                 (append! lines (py-statement-with-cells expr indent cell-vars)))))))))