Collapse signal platform primitives into pure SX dicts

Replace _Signal class (Python) and SxSignal constructor (JS) with plain
dicts keyed by "__signal". Nine platform accessor functions become ~20
lines of pure SX in signals.sx. type-of returns "dict" for signals;
signal? is now a structural predicate (dict? + has-key?).

Net: -168 lines platform, +120 lines SX. Zero platform primitives for
reactivity — signals compile to any host via the bootstrappers.

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

shared/sx/ref/platform_js.py

@@ -851,13 +851,6 @@ PREAMBLE = '''\
   }
   Island.prototype._island = true;
 
-  function SxSignal(value) {
-    this.value = value;
-    this.subscribers = [];
-    this.deps = [];
-  }
-  SxSignal.prototype._signal = true;
-
   function Macro(params, restParam, body, closure, name) {
     this.params = params;
     this.restParam = restParam;
@@ -1141,7 +1134,6 @@ PLATFORM_JS_PRE = '''
     if (x._lambda) return "lambda";
     if (x._component) return "component";
     if (x._island) return "island";
-    if (x._signal) return "signal";
     if (x._spread) return "spread";
     if (x._macro) return "macro";
     if (x._raw) return "raw-html";
@@ -1252,17 +1244,6 @@ PLATFORM_JS_PRE = '''
     return new Island(name, params, hasChildren, body, merge(env));
   }
 
-  // Signal platform
-  function makeSignal(value) { return new SxSignal(value); }
-  function isSignal(x) { return x != null && x._signal === true; }
-  function signalValue(s) { return s.value; }
-  function signalSetValue(s, v) { s.value = v; }
-  function signalSubscribers(s) { return s.subscribers.slice(); }
-  function signalAddSub(s, fn) { if (s.subscribers.indexOf(fn) < 0) s.subscribers.push(fn); }
-  function signalRemoveSub(s, fn) { var i = s.subscribers.indexOf(fn); if (i >= 0) s.subscribers.splice(i, 1); }
-  function signalDeps(s) { return s.deps.slice(); }
-  function signalSetDeps(s, deps) { s.deps = Array.isArray(deps) ? deps.slice() : []; }
-
   // invoke — call any callable (native fn or SX lambda) with args.
   // Transpiled code emits direct calls f(args) which fail on SX lambdas
   // from runtime-evaluated island bodies. invoke dispatches correctly.
@@ -1376,6 +1357,9 @@ PLATFORM_JS_POST = '''
   }
   function mapDict(fn, d) { var r = {}; for (var k in d) r[k] = fn(k, d[k]); return r; }
 
+  // Predicate aliases used by transpiled code
+  var isDict = PRIMITIVES["dict?"];
+
   // List primitives used directly by transpiled code
   var len = PRIMITIVES["len"];
   var first = PRIMITIVES["first"];