Replace invoke with cek-call in reactive island primitives

All signal operations (computed, effect, batch, etc.) now dispatch
function calls through cek-call, which routes SX lambdas via cek-run
and native callables via apply. This replaces the invoke shim.

Key changes:
- cek.sx: add cek-call (defined before reactive-shift-deref), replace
  invoke in subscriber disposal and ReactiveResetFrame handler
- signals.sx: replace all 11 invoke calls with cek-call
- js.sx: fix octal escape in js-quote-string (char-from-code 0)
- platform_js.py: fix JS append to match Python (list concat semantics),
  add Continuation type guard in PLATFORM_CEK_JS, add scheduleIdle
  safety check, module ordering (cek before signals)
- platform_py.py: fix ident-char regex (remove [ ] from valid chars),
  module ordering (cek before signals)
- run_js_sx.py: emit PLATFORM_CEK_JS before transpiled spec files
- page-functions.sx: add cek and provide page functions for SX URLs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/sx/types.py

@@ -401,6 +401,43 @@ class EvalError(Exception):
     pass
 
 
+# ---------------------------------------------------------------------------
+# SxExpr
+# ---------------------------------------------------------------------------
+
+class SxExpr(str):
+    """Pre-built sx source that serialize() outputs unquoted.
+
+    ``SxExpr`` is a ``str`` subclass, so it works everywhere a plain
+    string does (join, startswith, f-strings, isinstance checks).  The
+    only difference: ``serialize()`` emits it unquoted instead of
+    wrapping it in double-quotes.
+
+    Use this to nest sx call strings inside other sx_call() invocations
+    without them being quoted as strings::
+
+        sx_call("parent", child=sx_call("child", x=1))
+        # => (~parent :child (~child :x 1))
+    """
+
+    def __new__(cls, source: str = "") -> "SxExpr":
+        return str.__new__(cls, source)
+
+    @property
+    def source(self) -> str:
+        """The raw SX source string (backward compat)."""
+        return str.__str__(self)
+
+    def __repr__(self) -> str:
+        return f"SxExpr({str.__repr__(self)})"
+
+    def __add__(self, other: object) -> "SxExpr":
+        return SxExpr(str.__add__(self, str(other)))
+
+    def __radd__(self, other: object) -> "SxExpr":
+        return SxExpr(str.__add__(str(other), self))
+
+
 # ---------------------------------------------------------------------------
 # Type alias
 # ---------------------------------------------------------------------------