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/bootstrap_py.py

@@ -165,12 +165,6 @@ class PyEmitter:
             "signal-remove-sub!": "signal_remove_sub",
             "signal-deps": "signal_deps",
             "signal-set-deps!": "signal_set_deps",
-            "set-tracking-context!": "set_tracking_context",
-            "get-tracking-context": "get_tracking_context",
-            "make-tracking-context": "make_tracking_context",
-            "tracking-context-deps": "tracking_context_deps",
-            "tracking-context-add-dep!": "tracking_context_add_dep",
-            "tracking-context-notify-fn": "tracking_context_notify_fn",
             "identical?": "is_identical",
             "notify-subscribers": "notify_subscribers",
             "flush-subscribers": "flush_subscribers",
@@ -179,7 +173,6 @@ class PyEmitter:
             "register-in-scope": "register_in_scope",
             "*batch-depth*": "_batch_depth",
             "*batch-queue*": "_batch_queue",
-            "*island-scope*": "_island_scope",
             "*store-registry*": "_store_registry",
             "def-store": "def_store",
             "use-store": "use_store",