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>

web/forms.sx

@@ -98,7 +98,7 @@
           (params   (get parsed "params"))
           (body     (get parsed "body")))
       (let ((hdef (make-handler-def name params body env opts)))
-        (env-set! env (str "handler:" name) hdef)
+        (env-bind! env (str "handler:" name) hdef)
         hdef))))
 
 
@@ -117,7 +117,7 @@
           (doc        (if has-doc (nth args 2) ""))
           (body       (if has-doc (nth args 3) (nth args 2))))
       (let ((qdef (make-query-def name params doc body env)))
-        (env-set! env (str "query:" name) qdef)
+        (env-bind! env (str "query:" name) qdef)
         qdef))))
 
 
@@ -135,7 +135,7 @@
           (doc        (if has-doc (nth args 2) ""))
           (body       (if has-doc (nth args 3) (nth args 2))))
       (let ((adef (make-action-def name params doc body env)))
-        (env-set! env (str "action:" name) adef)
+        (env-bind! env (str "action:" name) adef)
         adef))))
 
 
@@ -163,7 +163,7 @@
                            (nth args (+ idx 1))))))
           (range 1 max-i 2)))
       (let ((pdef (make-page-def name slots env)))
-        (env-set! env (str "page:" name) pdef)
+        (env-bind! env (str "page:" name) pdef)
         pdef))))
 
 
@@ -266,7 +266,7 @@
           (bindings (stream-chunk-bindings chunk)))
       (for-each
         (fn ((key :as string))
-          (env-set! env (normalize-binding-key key)
+          (env-bind! env (normalize-binding-key key)
                     (get bindings key)))
         (keys bindings))
       env)))