Implement explicit CEK machine, continuations, effect signatures, fix dynamic-wind and inspect shadowing

Three-phase foundations implementation:

Phase A — Activate dormant shift/reset continuations with 24 SX-native tests
covering basic semantics, predicates, stored continuations, nested reset,
scope interaction, and TCO.

Phase B — Bridge compile-time effect system to runtime: boundary_parser extracts
46 effect annotations, platform provides populate_effect_annotations() and
check_component_effects() for static analysis. 6 new type tests.

Phase C — Explicit CEK machine (frames.sx + cek.sx): evaluation state as data
({control, env, kont, phase, value}), 21 frame types, two-phase step function
(step-eval/step-continue), native shift/reset via frame capture. Bootstrapper
integration: --spec-modules cek transpiles to Python with iterative cek_run.
43 interpreted + 49 transpiled tests passing.

Bug fixes:
- inspect() shadowed by `import inspect` in PLATFORM_ASYNC_PY — renamed to
  `import inspect as _inspect`
- dynamic-wind missing platform functions (call_thunk, push_wind!, pop_wind!) —
  added with try/finally error safety via dynamic_wind_call

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

shared/sx/types.py

@@ -361,11 +361,15 @@ class Continuation:
 
     Callable with one argument — provides the value that the shift
     expression "returns" within the delimited context.
+
+    _cek_data: optional dict with CEK frame data (captured frames, rest-kont)
+    for continuations created by the explicit CEK machine.
     """
-    __slots__ = ("fn",)
+    __slots__ = ("fn", "_cek_data")
 
     def __init__(self, fn):
         self.fn = fn
+        self._cek_data = None
 
     def __call__(self, value=NIL):
         return self.fn(value)