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/javascript/platform.py

@@ -1162,12 +1162,12 @@ PLATFORM_JS_PRE = '''
   function makeSymbol(n) { return new Symbol(n); }
   function makeKeyword(n) { return new Keyword(n); }
 
-  function makeLambda(params, body, env) { return new Lambda(params, body, merge(env)); }
+  function makeLambda(params, body, env) { return new Lambda(params, body, env); }
   function makeComponent(name, params, hasChildren, body, env, affinity) {
-    return new Component(name, params, hasChildren, body, merge(env), affinity);
+    return new Component(name, params, hasChildren, body, env, affinity);
   }
   function makeMacro(params, restParam, body, env, name) {
-    return new Macro(params, restParam, body, merge(env), name);
+    return new Macro(params, restParam, body, env, name);
   }
   function makeThunk(expr, env) { return new Thunk(expr, env); }
 
@@ -1257,7 +1257,7 @@ PLATFORM_JS_PRE = '''
 
   // Island platform
   function makeIsland(name, params, hasChildren, body, env) {
-    return new Island(name, params, hasChildren, body, merge(env));
+    return new Island(name, params, hasChildren, body, env);
   }
 
   // JSON / dict helpers for island state serialization
@@ -1272,6 +1272,11 @@ PLATFORM_JS_PRE = '''
 
   function envHas(env, name) { return name in env; }
   function envGet(env, name) { return env[name]; }
+  function envBind(env, name, val) {
+    // Direct property set — creates or overwrites on THIS env only.
+    // Used by let, define, defcomp, lambda param binding.
+    env[name] = val;
+  }
   function envSet(env, name, val) {
     // Walk prototype chain to find where the variable is defined (for set!)
     var obj = env;