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_js.py

@@ -174,6 +174,8 @@ class JSEmitter:
             "sf-quasiquote": "sfQuasiquote",
             "sf-thread-first": "sfThreadFirst",
             "sf-set!": "sfSetBang",
+            "sf-reset": "sfReset",
+            "sf-shift": "sfShift",
             "qq-expand": "qqExpand",
             "ho-map": "hoMap",
             "ho-map-indexed": "hoMapIndexed",
@@ -1162,6 +1164,16 @@ PREAMBLE = '''\
   }
   StyleValue.prototype._styleValue = true;
 
+  function Continuation(fn) { this.fn = fn; }
+  Continuation.prototype._continuation = true;
+  Continuation.prototype.call = function(value) { return this.fn(value !== undefined ? value : NIL); };
+
+  function ShiftSignal(kName, body, env) {
+    this.kName = kName;
+    this.body = body;
+    this.env = env;
+  }
+
   function isSym(x) { return x != null && x._sym === true; }
   function isKw(x) { return x != null && x._kw === true; }
 
@@ -1199,6 +1211,7 @@ PLATFORM_JS = '''
     if (x._macro) return "macro";
     if (x._raw) return "raw-html";
     if (x._styleValue) return "style-value";
+    if (x._continuation) return "continuation";
     if (typeof Node !== "undefined" && x instanceof Node) return "dom-node";
     if (Array.isArray(x)) return "list";
     if (typeof x === "object") return "dict";
@@ -1370,6 +1383,7 @@ PLATFORM_JS = '''
   PRIMITIVES["string?"] = function(x) { return typeof x === "string"; };
   PRIMITIVES["list?"] = Array.isArray;
   PRIMITIVES["dict?"] = function(x) { return x !== null && typeof x === "object" && !Array.isArray(x) && !x._sym && !x._kw; };
+  PRIMITIVES["continuation?"] = function(x) { return x != null && x._continuation === true; };
   PRIMITIVES["empty?"] = function(c) { return isNil(c) || (Array.isArray(c) ? c.length === 0 : typeof c === "string" ? c.length === 0 : Object.keys(c).length === 0); };
   PRIMITIVES["contains?"] = function(c, k) {
     if (typeof c === "string") return c.indexOf(String(k)) !== -1;
@@ -1559,7 +1573,7 @@ PLATFORM_JS = '''
     "if":1,"when":1,"cond":1,"case":1,"and":1,"or":1,"let":1,"let*":1,
     "lambda":1,"fn":1,"define":1,"defcomp":1,"defmacro":1,"defstyle":1,
     "defkeyframes":1,"defhandler":1,"begin":1,"do":1,
-    "quote":1,"quasiquote":1,"->":1,"set!":1
+    "quote":1,"quasiquote":1,"->":1,"set!":1,"reset":1,"shift":1
   }; }
   function isHoForm(n) { return n in {
     "map":1,"map-indexed":1,"filter":1,"reduce":1,"some":1,"every?":1,"for-each":1
@@ -2639,6 +2653,44 @@ def fixups_js(has_html, has_sx, has_dom):
     return _rawCallLambda(f, args, callerEnv);
   };
 
+  // =========================================================================
+  // Delimited continuations (shift/reset)
+  // =========================================================================
+  var _resetResume = [];  // stack of resume values
+
+  function sfReset(args, env) {
+    var body = args[0];
+    try {
+      return trampoline(evalExpr(body, env));
+    } catch (e) {
+      if (e instanceof ShiftSignal) {
+        var sig = e;
+        var cont = new Continuation(function(value) {
+          if (value === undefined) value = NIL;
+          _resetResume.push(value);
+          try {
+            return trampoline(evalExpr(body, env));
+          } finally {
+            _resetResume.pop();
+          }
+        });
+        var sigEnv = merge(sig.env);
+        sigEnv[sig.kName] = cont;
+        return trampoline(evalExpr(sig.body, sigEnv));
+      }
+      throw e;
+    }
+  }
+
+  function sfShift(args, env) {
+    if (_resetResume.length > 0) {
+      return _resetResume[_resetResume.length - 1];
+    }
+    var kName = symbolName(args[0]);
+    var body = args[1];
+    throw new ShiftSignal(kName, body, env);
+  }
+
   // Expose render functions as primitives so SX code can call them''']
     if has_html:
         lines.append('  if (typeof renderToHtml === "function") PRIMITIVES["render-to-html"] = renderToHtml;')