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_py.py

@@ -225,8 +225,6 @@ def type_of(x):
         return "component"
     if isinstance(x, Island):
         return "island"
-    if isinstance(x, _Signal):
-        return "signal"
     if isinstance(x, _Spread):
         return "spread"
     if isinstance(x, Macro):
@@ -468,58 +466,6 @@ def is_identical(a, b):
     return a is b
 
 
-# -------------------------------------------------------------------------
-# Signal platform -- reactive state primitives
-# -------------------------------------------------------------------------
-
-class _Signal:
-    """Reactive signal container."""
-    __slots__ = ("value", "subscribers", "deps")
-    def __init__(self, value):
-        self.value = value
-        self.subscribers = []
-        self.deps = []
-
-
-def make_signal(value):
-    return _Signal(value)
-
-
-def is_signal(x):
-    return isinstance(x, _Signal)
-
-
-def signal_value(s):
-    return s.value if isinstance(s, _Signal) else s
-
-
-def signal_set_value(s, v):
-    if isinstance(s, _Signal):
-        s.value = v
-
-
-def signal_subscribers(s):
-    return list(s.subscribers) if isinstance(s, _Signal) else []
-
-
-def signal_add_sub(s, fn):
-    if isinstance(s, _Signal) and fn not in s.subscribers:
-        s.subscribers.append(fn)
-
-
-def signal_remove_sub(s, fn):
-    if isinstance(s, _Signal) and fn in s.subscribers:
-        s.subscribers.remove(fn)
-
-
-def signal_deps(s):
-    return list(s.deps) if isinstance(s, _Signal) else []
-
-
-def signal_set_deps(s, deps):
-    if isinstance(s, _Signal):
-        s.deps = list(deps) if isinstance(deps, list) else []
-
 
 def invoke(f, *args):
     """Call f with args — handles both native callables and SX lambdas.
@@ -1117,6 +1063,7 @@ replace = PRIMITIVES["replace"]
 parse_int = PRIMITIVES["parse-int"]
 upper = PRIMITIVES["upper"]
 has_key_p = PRIMITIVES["has-key?"]
+dict_p = PRIMITIVES["dict?"]
 dissoc = PRIMITIVES["dissoc"]
 index_of = PRIMITIVES["index-of"]
 '''