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()