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/ref/bootstrap_py.py

@@ -186,6 +186,8 @@ class PyEmitter:
             "sf-quasiquote": "sf_quasiquote",
             "sf-thread-first": "sf_thread_first",
             "sf-set!": "sf_set_bang",
+            "sf-reset": "sf_reset",
+            "sf-shift": "sf_shift",
             "qq-expand": "qq_expand",
             "ho-map": "ho_map",
             "ho-map-indexed": "ho_map_indexed",
@@ -887,8 +889,8 @@ from typing import Any
 # =========================================================================
 
 from shared.sx.types import (
-    NIL, Symbol, Keyword, Lambda, Component, Macro, StyleValue,
-    HandlerDef, QueryDef, ActionDef, PageDef,
+    NIL, Symbol, Keyword, Lambda, Component, Continuation, Macro, StyleValue,
+    HandlerDef, QueryDef, ActionDef, PageDef, _ShiftSignal,
 )
 from shared.sx.parser import SxExpr
 '''
@@ -998,6 +1000,8 @@ def type_of(x):
         return "raw-html"
     if isinstance(x, StyleValue):
         return "style-value"
+    if isinstance(x, Continuation):
+        return "continuation"
     if isinstance(x, list):
         return "list"
     if isinstance(x, dict):
@@ -1338,7 +1342,7 @@ _SPECIAL_FORM_NAMES = frozenset([
     "define", "defcomp", "defmacro", "defstyle", "defkeyframes",
     "defhandler", "defpage", "defquery", "defaction", "defrelation",
     "begin", "do", "quote", "quasiquote",
-    "->", "set!",
+    "->", "set!", "reset", "shift",
 ])
 
 _HO_FORM_NAMES = frozenset([
@@ -1501,6 +1505,11 @@ def aser_special(name, expr, env):
                 "defhandler", "defpage", "defquery", "defaction", "defrelation"):
         trampoline(eval_expr(expr, env))
         return NIL
+    # reset/shift — evaluate normally in aser mode (they're control flow)
+    if name == "reset":
+        return sf_reset(args, env)
+    if name == "shift":
+        return sf_shift(args, env)
     # Lambda/fn, quote, quasiquote, set!, -> : evaluate normally
     result = eval_expr(expr, env)
     return trampoline(result)
@@ -1587,6 +1596,7 @@ PRIMITIVES["number?"] = lambda x: isinstance(x, (int, float)) and not isinstance
 PRIMITIVES["string?"] = lambda x: isinstance(x, str)
 PRIMITIVES["list?"] = lambda x: isinstance(x, _b_list)
 PRIMITIVES["dict?"] = lambda x: isinstance(x, _b_dict)
+PRIMITIVES["continuation?"] = lambda x: isinstance(x, Continuation)
 PRIMITIVES["empty?"] = lambda c: (
     c is None or c is NIL or
     (isinstance(c, (_b_list, str, _b_dict)) and _b_len(c) == 0)
@@ -1725,6 +1735,38 @@ concat = PRIMITIVES["concat"]
 '''
 
 FIXUPS_PY = '''
+# =========================================================================
+# Delimited continuations (shift/reset)
+# =========================================================================
+
+_RESET_RESUME = []  # stack of resume values; empty = not resuming
+
+def sf_reset(args, env):
+    """(reset body) -- establish a continuation delimiter."""
+    body = first(args)
+    try:
+        return trampoline(eval_expr(body, env))
+    except _ShiftSignal as sig:
+        def cont_fn(value=NIL):
+            _RESET_RESUME.append(value)
+            try:
+                return trampoline(eval_expr(body, env))
+            finally:
+                _RESET_RESUME.pop()
+        k = Continuation(cont_fn)
+        sig_env = dict(sig.env)
+        sig_env[sig.k_name] = k
+        return trampoline(eval_expr(sig.body, sig_env))
+
+def sf_shift(args, env):
+    """(shift k body) -- capture continuation to nearest reset."""
+    if _RESET_RESUME:
+        return _RESET_RESUME[-1]
+    k_name = symbol_name(first(args))
+    body = nth(args, 1)
+    raise _ShiftSignal(k_name, body, env)
+
+
 # =========================================================================
 # Fixups -- wire up render adapter dispatch
 # =========================================================================