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>

sx/sx/docs-content.sx

@@ -22,7 +22,7 @@
     (~doc-section :title "What sx is not" :id "not"
       (ul :class "space-y-2 text-stone-600"
         (li "Not a general-purpose programming language — it's a UI rendering language")
-        (li "Not a full Lisp — it has macros and TCO, but no continuations or call/cc")
+        (li "Not a full Lisp — it has macros, TCO, and delimited continuations, but no full call/cc")
         (li "Not production-hardened at scale — it runs one website")))))
 
 (defcomp ~docs-getting-started-content ()