Fix lambda multi-body, reactive island demos, and add React is Hypermedia essay

Lambda multi-body fix: sf-lambda used (nth args 1), dropping all but the first
body expression. Fixed to collect all body expressions and wrap in (begin ...).
This was foundational — every multi-expression lambda in every island silently
dropped expressions after the first.

Reactive islands: fix dom-parent marker timing (first effect run before marker
is in DOM), fix :key eager evaluation, fix error boundary scope isolation,
fix resource/suspense reactive cond tracking, fix inc not available as JS var.

New essay: "React is Hypermedia" — argues that reactive islands are hypermedia
controls whose behavior is specified in SX, not a departure from hypermedia.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/bootstrap_js.py

@@ -102,9 +102,11 @@ class JSEmitter:
         # Map SX names to JS names
         return self._mangle(name)
 
-    def _mangle(self, name: str) -> str:
-        """Convert SX identifier to valid JS identifier."""
-        RENAMES = {
+    # Explicit SX→JS name mappings. Auto-mangle (kebab→camelCase, ?→_p, !→_b)
+    # is only a fallback. Every platform symbol used in spec .sx files MUST have
+    # an entry here — relying on auto-mangle is fragile and has caused runtime
+    # errors (e.g. has-key? → hasKey_p instead of dictHas).
+    RENAMES = {
             "nil": "NIL",
             "true": "true",
             "false": "false",
@@ -328,6 +330,7 @@ class JSEmitter:
             "dom-listen": "domListen",
             "event-detail": "eventDetail",
             "dom-query": "domQuery",
+            "dom-ensure-element": "domEnsureElement",
             "dom-query-all": "domQueryAll",
             "dom-tag-name": "domTagName",
             "create-comment": "createComment",
@@ -338,6 +341,7 @@ class JSEmitter:
             "dom-get-data": "domGetData",
             "json-parse": "jsonParse",
             "dict-has?": "dictHas",
+            "has-key?": "dictHas",
             "dict-delete!": "dictDelete",
             "process-bindings": "processBindings",
             "eval-cond": "evalCond",
@@ -413,6 +417,7 @@ class JSEmitter:
             "promise-resolve": "promiseResolve",
             "promise-then": "promiseThen",
             "promise-catch": "promiseCatch",
+            "promise-delayed": "promiseDelayed",
             "abort-previous": "abortPrevious",
             "track-controller": "trackController",
             "new-abort-controller": "newAbortController",
@@ -590,9 +595,12 @@ class JSEmitter:
             "match-route": "matchRoute",
             "find-matching-route": "findMatchingRoute",
             "for-each-indexed": "forEachIndexed",
-        }
-        if name in RENAMES:
-            return RENAMES[name]
+    }
+
+    def _mangle(self, name: str) -> str:
+        """Convert SX identifier to valid JS identifier."""
+        if name in self.RENAMES:
+            return self.RENAMES[name]
         # General mangling: replace - with camelCase, ? with _p, ! with _b
         result = name
         if result.endswith("?"):
@@ -2196,6 +2204,7 @@ PRIMITIVES_JS_MODULES: dict[str, str] = {
   PRIMITIVES["odd?"] = function(n) { return n % 2 !== 0; };
   PRIMITIVES["even?"] = function(n) { return n % 2 === 0; };
   PRIMITIVES["zero?"] = function(n) { return n === 0; };
+  PRIMITIVES["boolean?"] = function(x) { return x === true || x === false; };
 ''',
 
     "core.strings": '''
@@ -2219,6 +2228,7 @@ PRIMITIVES_JS_MODULES: dict[str, str] = {
   PRIMITIVES["slice"] = function(c, a, b) { return b !== undefined ? c.slice(a, b) : c.slice(a); };
   PRIMITIVES["substring"] = function(s, a, b) { return String(s).substring(a, b); };
   PRIMITIVES["string-length"] = function(s) { return String(s).length; };
+  PRIMITIVES["string-contains?"] = function(s, sub) { return String(s).indexOf(String(sub)) !== -1; };
   PRIMITIVES["concat"] = function() {
     var out = [];
     for (var i = 0; i < arguments.length; i++) if (!isNil(arguments[i])) out = out.concat(arguments[i]);
@@ -2254,6 +2264,12 @@ PRIMITIVES_JS_MODULES: dict[str, str] = {
   PRIMITIVES["zip-pairs"] = function(c) {
     var r = []; for (var i = 0; i < c.length - 1; i++) r.push([c[i], c[i + 1]]); return r;
   };
+  PRIMITIVES["reverse"] = function(c) { return Array.isArray(c) ? c.slice().reverse() : String(c).split("").reverse().join(""); };
+  PRIMITIVES["flatten"] = function(c) {
+    var out = [];
+    function walk(a) { for (var i = 0; i < a.length; i++) Array.isArray(a[i]) ? walk(a[i]) : out.push(a[i]); }
+    walk(c || []); return out;
+  };
 ''',
 
     "core.dict": '''
@@ -2276,6 +2292,7 @@ PRIMITIVES_JS_MODULES: dict[str, str] = {
     return out;
   };
   PRIMITIVES["dict-set!"] = function(d, k, v) { d[k] = v; return v; };
+  PRIMITIVES["has-key?"] = function(d, k) { return d !== null && d !== undefined && k in d; };
   PRIMITIVES["into"] = function(target, coll) {
     if (Array.isArray(target)) return Array.isArray(coll) ? coll.slice() : Object.entries(coll);
     var r = {}; for (var i = 0; i < coll.length; i++) { var p = coll[i]; if (Array.isArray(p) && p.length >= 2) r[p[0]] = p[1]; }
@@ -2906,6 +2923,20 @@ PLATFORM_DOM_JS = """
     return _hasDom ? document.querySelector(sel) : null;
   }
 
+  function domEnsureElement(sel) {
+    if (!_hasDom) return null;
+    var el = document.querySelector(sel);
+    if (el) return el;
+    // Parse #id selector → create div with that id, append to body
+    if (sel.charAt(0) === '#') {
+      el = document.createElement('div');
+      el.id = sel.slice(1);
+      document.body.appendChild(el);
+      return el;
+    }
+    return null;
+  }
+
   function domQueryAll(root, sel) {
     if (!root || !root.querySelectorAll) return [];
     return Array.prototype.slice.call(root.querySelectorAll(sel));
@@ -3039,6 +3070,12 @@ PLATFORM_ORCHESTRATION_JS = """
 
   function promiseCatch(p, fn) { return p && p.catch ? p.catch(fn) : p; }
 
+  function promiseDelayed(ms, value) {
+    return new Promise(function(resolve) {
+      setTimeout(function() { resolve(value); }, ms);
+    });
+  }
+
   // --- Abort controllers ---
 
   var _controllers = typeof WeakMap !== "undefined" ? new WeakMap() : null;
@@ -3075,8 +3112,9 @@ PLATFORM_ORCHESTRATION_JS = """
   function clearTimeout_(id) { clearTimeout(id); }
   function clearInterval_(id) { clearInterval(id); }
   function requestAnimationFrame_(fn) {
-    if (typeof requestAnimationFrame !== "undefined") requestAnimationFrame(fn);
-    else setTimeout(fn, 16);
+    var cb = _wrapSxFn(fn);
+    if (typeof requestAnimationFrame !== "undefined") requestAnimationFrame(cb);
+    else setTimeout(cb, 16);
   }
 
   // --- Fetch ---
@@ -3475,14 +3513,19 @@ PLATFORM_ORCHESTRATION_JS = """
   function stopPropagation_(e) { if (e && e.stopPropagation) e.stopPropagation(); }
   function domFocus(el) { if (el && el.focus) el.focus(); }
   function tryCatch(tryFn, catchFn) {
-    try { return tryFn(); } catch (e) { return catchFn(e); }
+    var t = _wrapSxFn(tryFn);
+    var c = catchFn && catchFn._lambda
+      ? function(e) { return trampoline(callLambda(catchFn, [e], lambdaClosure(catchFn))); }
+      : catchFn;
+    try { return t(); } catch (e) { return c(e); }
   }
   function errorMessage(e) {
     return e && e.message ? e.message : String(e);
   }
   function scheduleIdle(fn) {
-    if (typeof requestIdleCallback !== "undefined") requestIdleCallback(fn);
-    else setTimeout(fn, 0);
+    var cb = _wrapSxFn(fn);
+    if (typeof requestIdleCallback !== "undefined") requestIdleCallback(cb);
+    else setTimeout(cb, 0);
   }
   function elementValue(el) { return el && el.value !== undefined ? el.value : NIL; }
 
@@ -4028,11 +4071,24 @@ def fixups_js(has_html, has_sx, has_dom, has_signals=False):
   PRIMITIVES["dom-listen"] = domListen;
   PRIMITIVES["dom-dispatch"] = domDispatch;
   PRIMITIVES["event-detail"] = eventDetail;
+  PRIMITIVES["resource"] = resource;
+  PRIMITIVES["promise-delayed"] = promiseDelayed;
+  PRIMITIVES["promise-then"] = promiseThen;
   PRIMITIVES["def-store"] = defStore;
   PRIMITIVES["use-store"] = useStore;
   PRIMITIVES["emit-event"] = emitEvent;
   PRIMITIVES["on-event"] = onEvent;
-  PRIMITIVES["bridge-event"] = bridgeEvent;''')
+  PRIMITIVES["bridge-event"] = bridgeEvent;
+  // DOM primitives for island code
+  PRIMITIVES["dom-focus"] = domFocus;
+  PRIMITIVES["dom-tag-name"] = domTagName;
+  PRIMITIVES["dom-get-prop"] = domGetProp;
+  PRIMITIVES["stop-propagation"] = stopPropagation_;
+  PRIMITIVES["error-message"] = errorMessage;
+  PRIMITIVES["schedule-idle"] = scheduleIdle;
+  PRIMITIVES["invoke"] = invoke;
+  PRIMITIVES["error"] = function(msg) { throw new Error(msg); };
+  PRIMITIVES["filter"] = filter;''')
     return "\n".join(lines)