Remove invoke from platform interfaces, add cek-call integration tests

- platform_js.py: remove invoke function definition and PRIMITIVES
  registration, switch domListen handler wrapping to cek-call
- platform_py.py: remove invoke function definition
- run_signal_tests.py: remove invoke patch, use cek_call in batch wrapper
- run_cek_reactive_tests.py: remove invoke, fix primitive lookup to use
  two-level is_primitive/get_primitive, increase recursion limit for
  interpreted CEK evaluation
- test-cek-reactive.sx: add 7 new tests covering cek-call dispatch with
  computed, effect, cleanup, batch coalescing

All 79 tests pass (20 signal + 43 CEK + 16 CEK reactive).

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

shared/sx/ref/platform_py.py

@@ -467,15 +467,6 @@ def is_identical(a, b):
 
 
 
-def invoke(f, *args):
-    """Call f with args — handles both native callables and SX lambdas.
-
-    In Python, all transpiled lambdas are natively callable, so this is
-    just a direct call. The JS host needs dispatch logic here because
-    SX lambdas from runtime-evaluated code are objects, not functions.
-    """
-    return f(*args)
-
 
 def json_serialize(obj):
     import json