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/boot.sx

@@ -361,7 +361,7 @@
               ;; Bind params from kwargs
               (for-each
                 (fn ((p :as string))
-                  (env-set! local p (if (dict-has? kwargs p) (dict-get kwargs p) nil)))
+                  (env-bind! local p (if (dict-has? kwargs p) (dict-get kwargs p) nil)))
                 (component-params comp))
 
               ;; Render the island body in a reactive scope