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/types.py

@@ -168,7 +168,7 @@ class Component:
     closure: dict[str, Any] = field(default_factory=dict)
     css_classes: set[str] = field(default_factory=set)  # pre-scanned :class values
     deps: set[str] = field(default_factory=set)        # transitive component deps (~names)
-    io_refs: set[str] = field(default_factory=set)     # transitive IO primitive refs
+    io_refs: set[str] | None = None     # transitive IO primitive refs (None = not computed)
     affinity: str = "auto"    # "auto" | "client" | "server"
 
     @property
@@ -208,7 +208,7 @@ class Island:
     closure: dict[str, Any] = field(default_factory=dict)
     css_classes: set[str] = field(default_factory=set)
     deps: set[str] = field(default_factory=set)
-    io_refs: set[str] = field(default_factory=set)
+    io_refs: set[str] | None = None
 
     def __repr__(self):
         return f"<Island ~{self.name}({', '.join(self.params)})>"