Fix all 9 spec test failures: Env scope chain, IO detection, offline mutation

- env.py: Add MergedEnv with dual-parent lookup (primary for set!,
  secondary for reads), add dict-compat methods to Env
- platform_py.py: make_lambda stores env reference (no copy), env_merge
  uses MergedEnv for proper set! propagation, ancestor detection prevents
  unbounded chains in TCO recursion, sf_set_bang walks scope chain
- types.py: Component/Island io_refs defaults to None (not computed)
  instead of empty set, so component-pure? falls through to scan
- run.py: Test env uses Env class, mock execute-action calls SX lambdas
  via _call_sx instead of direct Python call

Spec tests: 320/320 (was 311/320)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/tests/run.py

@@ -281,7 +281,9 @@ def eval_file(filename, env):
 
 
 # --- Build env ---
-env = {
+from shared.sx.env import Env as _Env
+
+env = _Env({
     "try-call": try_call,
     "report-pass": report_pass,
     "report-fail": report_fail,
@@ -330,7 +332,7 @@ env = {
     "component-body": lambda c: getattr(c, 'body', NIL),
     "component-closure": lambda c: dict(getattr(c, 'closure', {})),
     "component-has-children?": lambda c: getattr(c, 'has_children', False),
-}
+})
 
 
 def _call_sx(fn, args, caller_env):
@@ -496,7 +498,7 @@ def _load_orchestration(env):
 
     def _mock_execute_action(action, payload, on_success, on_error):
         """Mock: immediately calls on_success with payload as 'server truth'."""
-        on_success(payload)
+        _call_sx(on_success, [payload], env)
         return NIL
 
     def _dict_delete(d, k):