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_py.py

@@ -481,17 +481,6 @@ class _Signal:
         self.deps = []
 
 
-class _TrackingContext:
-    """Context for discovering signal dependencies."""
-    __slots__ = ("notify_fn", "deps")
-    def __init__(self, notify_fn):
-        self.notify_fn = notify_fn
-        self.deps = []
-
-
-_tracking_context = None
-
-
 def make_signal(value):
     return _Signal(value)
 
@@ -532,33 +521,6 @@ def signal_set_deps(s, deps):
         s.deps = list(deps) if isinstance(deps, list) else []
 
 
-def set_tracking_context(ctx):
-    global _tracking_context
-    _tracking_context = ctx
-
-
-def get_tracking_context():
-    global _tracking_context
-    return _tracking_context if _tracking_context is not None else NIL
-
-
-def make_tracking_context(notify_fn):
-    return _TrackingContext(notify_fn)
-
-
-def tracking_context_deps(ctx):
-    return ctx.deps if isinstance(ctx, _TrackingContext) else []
-
-
-def tracking_context_add_dep(ctx, s):
-    if isinstance(ctx, _TrackingContext) and s not in ctx.deps:
-        ctx.deps.append(s)
-
-
-def tracking_context_notify_fn(ctx):
-    return ctx.notify_fn if isinstance(ctx, _TrackingContext) else NIL
-
-
 def invoke(f, *args):
     """Call f with args — handles both native callables and SX lambdas.