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/tests/test-cek-reactive.sx

@@ -39,7 +39,7 @@
 (defsuite "deref signal without reactive-reset"
   (deftest "deref signal returns current value"
     (let ((s (signal 99)))
-      (env-set! (test-env) "test-sig" s)
+      (env-bind! (test-env) "test-sig" s)
       (let ((result (eval-expr-cek
                       (sx-parse-one "(deref test-sig)")
                       (test-env))))
@@ -47,7 +47,7 @@
 
   (deftest "deref signal in expression returns computed value"
     (let ((s (signal 10)))
-      (env-set! (test-env) "test-sig" s)
+      (env-bind! (test-env) "test-sig" s)
       (let ((result (eval-expr-cek
                       (sx-parse-one "(+ 5 (deref test-sig))")
                       (test-env))))
@@ -67,7 +67,7 @@
                       (make-cek-state
                         (sx-parse-one "(deref test-sig)")
                         (let ((e (env-extend (test-env))))
-                          (env-set! e "test-sig" s)
+                          (env-bind! e "test-sig" s)
                           e)
                         (list (make-reactive-reset-frame
                                 (test-env)
@@ -83,7 +83,7 @@
       ;; Set up reactive-reset with tracking update-fn
       (scope-push! "sx-island-scope" nil)
       (let ((e (env-extend (test-env))))
-        (env-set! e "test-sig" s)
+        (env-bind! e "test-sig" s)
         (cek-run
           (make-cek-state
             (sx-parse-one "(deref test-sig)")
@@ -107,7 +107,7 @@
           (update-calls (list)))
       (scope-push! "sx-island-scope" nil)
       (let ((e (env-extend (test-env))))
-        (env-set! e "test-sig" s)
+        (env-bind! e "test-sig" s)
         ;; (str "val=" (deref test-sig)) — continuation captures (str "val=" [HOLE])
         (let ((result (cek-run
                         (make-cek-state
@@ -137,7 +137,7 @@
       ;; Create island scope with collector that accumulates disposers
       (scope-push! "sx-island-scope" (fn (d) (append! disposers d)))
       (let ((e (env-extend (test-env))))
-        (env-set! e "test-sig" s)
+        (env-bind! e "test-sig" s)
         (cek-run
           (make-cek-state
             (sx-parse-one "(deref test-sig)")
@@ -266,7 +266,7 @@
 
   (deftest "for-each through CEK"
     (let ((log (list)))
-      (env-set! (test-env) "test-log" log)
+      (env-bind! (test-env) "test-log" log)
       (eval-expr-cek
         (sx-parse-one "(for-each (fn (x) (append! test-log x)) (list 1 2 3))")
         (test-env))