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/js.sx

@@ -87,12 +87,6 @@
   "signal-remove-sub!" "signalRemoveSub"
   "signal-deps" "signalDeps"
   "signal-set-deps!" "signalSetDeps"
-  "set-tracking-context!" "setTrackingContext"
-  "get-tracking-context" "getTrackingContext"
-  "make-tracking-context" "makeTrackingContext"
-  "tracking-context-deps" "trackingContextDeps"
-  "tracking-context-add-dep!" "trackingContextAddDep"
-  "tracking-context-notify-fn" "trackingContextNotifyFn"
   "identical?" "isIdentical"
   "notify-subscribers" "notifySubscribers"
   "flush-subscribers" "flushSubscribers"
@@ -101,7 +95,6 @@
   "register-in-scope" "registerInScope"
   "*batch-depth*" "_batchDepth"
   "*batch-queue*" "_batchQueue"
-  "*island-scope*" "_islandScope"
   "*store-registry*" "_storeRegistry"
   "def-store" "defStore"
   "use-store" "useStore"