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/ref/platform_py.py

@@ -57,6 +57,7 @@ from shared.sx.types import (
     HandlerDef, QueryDef, ActionDef, PageDef, _ShiftSignal,
 )
 from shared.sx.parser import SxExpr
+from shared.sx.env import Env as _Env, MergedEnv as _MergedEnv
 '''
 
 # ---------------------------------------------------------------------------
@@ -195,8 +196,15 @@ def make_keyword(n):
     return Keyword(n)
 
 
+def _ensure_env(env):
+    """Wrap plain dict in Env if needed."""
+    if isinstance(env, _Env):
+        return env
+    return _Env(env if isinstance(env, dict) else {})
+
+
 def make_lambda(params, body, env):
-    return Lambda(params=list(params), body=body, closure=dict(env))
+    return Lambda(params=list(params), body=body, closure=_ensure_env(env))
 
 
 def make_component(name, params, has_children, body, env, affinity="auto"):
@@ -490,13 +498,26 @@ def env_set(env, name, val):
 
 
 def env_extend(env):
-    return dict(env)
+    return _ensure_env(env).extend()
 
 
 def env_merge(base, overlay):
-    result = dict(base)
-    result.update(overlay)
-    return result
+    base = _ensure_env(base)
+    overlay = _ensure_env(overlay)
+    if base is overlay:
+        # Same env — just extend with empty local scope for params
+        return base.extend()
+    # Check if base is an ancestor of overlay — if so, no need to merge
+    # (common for self-recursive calls where closure == caller's ancestor)
+    p = overlay
+    depth = 0
+    while p is not None and depth < 100:
+        if p is base:
+            return base.extend()
+        p = getattr(p, '_parent', None)
+        depth += 1
+    # MergedEnv: reads walk base then overlay; set! walks base only
+    return _MergedEnv({}, primary=base, secondary=overlay)
 
 
 def dict_set(d, k, v):
@@ -1022,8 +1043,10 @@ PLATFORM_DEPS_PY = (
     '    return list(classes)\n'
     '\n'
     'def component_io_refs(c):\n'
-    '    """Return cached IO refs list for a component (may be empty)."""\n'
-    '    return list(c.io_refs) if hasattr(c, "io_refs") and c.io_refs else []\n'
+    '    """Return cached IO refs list, or NIL if not yet computed."""\n'
+    '    if not hasattr(c, "io_refs") or c.io_refs is None:\n'
+    '        return NIL\n'
+    '    return list(c.io_refs)\n'
     '\n'
     'def component_set_io_refs(c, refs):\n'
     '    """Cache IO refs on a component."""\n'
@@ -1255,6 +1278,20 @@ def _wrap_aser_outputs():
         return SxExpr(result) if isinstance(result, str) else result
     aser_call = _aser_call_wrapped
     aser_fragment = _aser_fragment_wrapped
+
+
+# Override sf_set_bang to walk the Env scope chain so that (set! var val)
+# updates the variable in its defining scope, not just the local copy.
+def sf_set_bang(args, env):
+    name = symbol_name(first(args))
+    value = trampoline(eval_expr(nth(args, 1), env))
+    env = _ensure_env(env)
+    try:
+        env.set(name, value)
+    except KeyError:
+        # Not found in chain — define locally (matches prior behavior)
+        env[name] = value
+    return value
 '''
 
 # ---------------------------------------------------------------------------
@@ -1349,7 +1386,9 @@ def public_api_py(has_html: bool, has_sx: bool, has_deps: bool = False) -> str:
         'def evaluate(expr, env=None):',
         '    """Evaluate expr in env and return the result."""',
         '    if env is None:',
-        '        env = {}',
+        '        env = _Env()',
+        '    elif isinstance(env, dict):',
+        '        env = _Env(env)',
         '    result = eval_expr(expr, env)',
         '    while is_thunk(result):',
         '        result = eval_expr(thunk_expr(result), thunk_env(result))',
@@ -1369,8 +1408,8 @@ def public_api_py(has_html: bool, has_sx: bool, has_deps: bool = False) -> str:
         '',
         '',
         'def make_env(**kwargs):',
-        '    """Create an environment dict with initial bindings."""',
-        '    return dict(kwargs)',
+        '    """Create an environment with initial bindings."""',
+        '    return _Env(dict(kwargs))',
     ])
     return '\n'.join(lines)