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/primitives.py

@@ -187,6 +187,11 @@ def prim_is_list(x: Any) -> bool:
 def prim_is_dict(x: Any) -> bool:
     return isinstance(x, dict)
 
+@register_primitive("continuation?")
+def prim_is_continuation(x: Any) -> bool:
+    from .types import Continuation
+    return isinstance(x, Continuation)
+
 @register_primitive("empty?")
 def prim_is_empty(coll: Any) -> bool:
     if coll is None or coll is NIL: