Implement delimited continuations (shift/reset) across all evaluators

Bootstrap shift/reset to both Python and JS targets. The implementation
uses exception-based capture with re-evaluation: reset wraps in try/catch
for ShiftSignal, shift raises to the nearest reset, and continuation
invocation pushes a resume value and re-evaluates the body.

- Add Continuation type and _ShiftSignal to shared/sx/types.py
- Add sf_reset/sf_shift to hand-written evaluator.py
- Add async versions to async_eval.py
- Add shift/reset dispatch to eval.sx spec
- Bootstrap to Python: FIXUPS_PY with sf_reset/sf_shift, regenerate sx_ref.py
- Bootstrap to JS: Continuation/ShiftSignal types, sfReset/sfShift in fixups
- Add continuation? primitive to both bootstrappers and primitives.sx
- Allow callables (including Continuation) in hand-written HO map
- 44 unit tests (22 per evaluator) covering: passthrough, abort, invoke,
  double invoke, predicate, stored continuation, nested reset, practical patterns
- Update continuations essay to reflect implemented status with examples

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

shared/sx/async_eval.py

@@ -484,6 +484,48 @@ _ASYNC_SPECIAL_FORMS: dict[str, Any] = {
 }
 
 
+# ---------------------------------------------------------------------------
+# Async delimited continuations — shift / reset
+# ---------------------------------------------------------------------------
+
+_ASYNC_RESET_RESUME: list = []
+
+
+async def _asf_reset(expr, env, ctx):
+    """(reset body) — async version."""
+    from .types import Continuation, _ShiftSignal
+    body = expr[1]
+    try:
+        return await async_eval(body, env, ctx)
+    except _ShiftSignal as sig:
+        def cont_fn(value=None):
+            from .types import NIL
+            _ASYNC_RESET_RESUME.append(value if value is not None else NIL)
+            try:
+                # Sync re-evaluation; the async caller will trampoline
+                from .evaluator import _eval as sync_eval, _trampoline
+                return _trampoline(sync_eval(body, env))
+            finally:
+                _ASYNC_RESET_RESUME.pop()
+        k = Continuation(cont_fn)
+        sig_env = dict(sig.env)
+        sig_env[sig.k_name] = k
+        return await async_eval(sig.body, sig_env, ctx)
+
+
+async def _asf_shift(expr, env, ctx):
+    """(shift k body) — async version."""
+    from .types import _ShiftSignal
+    if _ASYNC_RESET_RESUME:
+        return _ASYNC_RESET_RESUME[-1]
+    k_name = expr[1].name
+    body = expr[2]
+    raise _ShiftSignal(k_name, body, env)
+
+_ASYNC_SPECIAL_FORMS["reset"] = _asf_reset
+_ASYNC_SPECIAL_FORMS["shift"] = _asf_shift
+
+
 # ---------------------------------------------------------------------------
 # Async higher-order forms
 # ---------------------------------------------------------------------------