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>

spec/cek.sx

@@ -396,7 +396,7 @@
       (let ((k (make-cek-continuation captured rest-kont)))
         ;; Evaluate shift body with k bound, continuation goes to rest-kont
         (let ((shift-env (env-extend env)))
-          (env-set! shift-env k-name k)
+          (env-bind! shift-env k-name k)
           (make-cek-state body shift-env rest-kont))))))
 
 
@@ -604,7 +604,7 @@
                     (body (get frame "body"))
                     (local (get frame "env")))
                 ;; Bind the value
-                (env-set! local name value)
+                (env-bind! local name value)
                 ;; More bindings?
                 (if (empty? remaining)
                   ;; All bindings done — evaluate body
@@ -628,7 +628,7 @@
                     (effect-list (get frame "effect-list")))
                 (when (and (lambda? value) (nil? (lambda-name value)))
                   (set-lambda-name! value name))
-                (env-set! fenv name value)
+                (env-bind! fenv name value)
                 ;; Effect annotation
                 (when has-effects
                   (let ((effect-names (if (= (type-of effect-list) "list")
@@ -640,7 +640,7 @@
                                        (env-get fenv "*effect-annotations*")
                                        (dict))))
                     (dict-set! effect-anns name effect-names)
-                    (env-set! fenv "*effect-annotations*" effect-anns)))
+                    (env-bind! fenv "*effect-annotations*" effect-anns)))
                 (make-cek-value value fenv rest-k))
 
             ;; --- SetFrame: value evaluated ---
@@ -969,10 +969,10 @@
                         " expects " (len params) " args, got " (len args)))
             (do
               (for-each
-                (fn (pair) (env-set! local (first pair) (nth pair 1)))
+                (fn (pair) (env-bind! local (first pair) (nth pair 1)))
                 (zip params args))
               (for-each
-                (fn (p) (env-set! local p nil))
+                (fn (p) (env-bind! local p nil))
                 (slice params (len args)))
               (make-cek-state (lambda-body f) local kont))))
 
@@ -983,10 +983,10 @@
               (children (nth parsed 1))
               (local (env-merge (component-closure f) env)))
           (for-each
-            (fn (p) (env-set! local p (or (dict-get kwargs p) nil)))
+            (fn (p) (env-bind! local p (or (dict-get kwargs p) nil)))
             (component-params f))
           (when (component-has-children? f)
-            (env-set! local "children" children))
+            (env-bind! local "children" children))
           (make-cek-state (component-body f) local kont))
 
       :else (error (str "Not callable: " (inspect f))))))