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/types.py

@@ -302,9 +302,45 @@ class StyleValue:
         return self.class_name
 
 
+# ---------------------------------------------------------------------------
+# Continuation
+# ---------------------------------------------------------------------------
+
+class Continuation:
+    """A captured delimited continuation (shift/reset).
+
+    Callable with one argument — provides the value that the shift
+    expression "returns" within the delimited context.
+    """
+    __slots__ = ("fn",)
+
+    def __init__(self, fn):
+        self.fn = fn
+
+    def __call__(self, value=NIL):
+        return self.fn(value)
+
+    def __repr__(self):
+        return "<continuation>"
+
+
+class _ShiftSignal(BaseException):
+    """Raised by shift to unwind to the nearest reset.
+
+    Inherits from BaseException (not Exception) to avoid being caught
+    by generic except clauses in user code.
+    """
+    __slots__ = ("k_name", "body", "env")
+
+    def __init__(self, k_name, body, env):
+        self.k_name = k_name
+        self.body = body
+        self.env = env
+
+
 # ---------------------------------------------------------------------------
 # Type alias
 # ---------------------------------------------------------------------------
 
 # An s-expression value after evaluation
-SExp = int | float | str | bool | Symbol | Keyword | Lambda | Macro | Component | HandlerDef | RelationDef | PageDef | QueryDef | ActionDef | StyleValue | list | dict | _Nil | None
+SExp = int | float | str | bool | Symbol | Keyword | Lambda | Macro | Component | Continuation | HandlerDef | RelationDef | PageDef | QueryDef | ActionDef | StyleValue | list | dict | _Nil | None