Collapse reactive islands into scopes: replace TrackingContext and *island-scope* with scope-push!/scope-pop!/context

Reactive tracking (deref/computed/effect dep discovery) and island lifecycle
now use the general scoped effects system instead of parallel infrastructure.
Two scope names: "sx-reactive" for tracking context, "sx-island-scope" for
island disposable collection. Eliminates ~98 net lines: _TrackingContext class,
7 tracking context platform functions (Python + JS), *island-scope* global,
and corresponding RENAME_MAP entries. All 20 signal tests pass (17 original +
3 new scope integration tests), plus CEK/continuation/type tests clean.

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

shared/sx/ref/platform_js.py

@@ -858,13 +858,6 @@ PREAMBLE = '''\
   }
   SxSignal.prototype._signal = true;
 
-  function TrackingCtx(notifyFn) {
-    this.notifyFn = notifyFn;
-    this.deps = [];
-  }
-
-  var _trackingContext = null;
-
   function Macro(params, restParam, body, closure, name) {
     this.params = params;
     this.restParam = restParam;
@@ -1269,12 +1262,6 @@ PLATFORM_JS_PRE = '''
   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() : []; }
-  function setTrackingContext(ctx) { _trackingContext = ctx; }
-  function getTrackingContext() { return _trackingContext || NIL; }
-  function makeTrackingContext(notifyFn) { return new TrackingCtx(notifyFn); }
-  function trackingContextDeps(ctx) { return ctx ? ctx.deps : []; }
-  function trackingContextAddDep(ctx, s) { if (ctx && ctx.deps.indexOf(s) < 0) ctx.deps.push(s); }
-  function trackingContextNotifyFn(ctx) { return ctx ? ctx.notifyFn : NIL; }
 
   // invoke — call any callable (native fn or SX lambda) with args.
   // Transpiled code emits direct calls f(args) which fail on SX lambdas