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/ref/eval.sx

@@ -941,14 +941,8 @@
     (let ((before (trampoline (eval-expr (first args) env)))
           (body   (trampoline (eval-expr (nth args 1) env)))
           (after  (trampoline (eval-expr (nth args 2) env))))
-      ;; Call entry thunk
-      (call-thunk before env)
-      ;; Push wind record, run body, pop, call exit
-      (push-wind! before after)
-      (let ((result (call-thunk body env)))
-        (pop-wind!)
-        (call-thunk after env)
-        result))))
+      ;; Delegate to platform — needs try/finally for error safety
+      (dynamic-wind-call before body after env))))
 
 
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