[core] Add TemplatableFn for 4-byte function-pointer templatable storage (#15545)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

tests/unit_tests/test_cpp_generator.py

@@ -669,11 +669,11 @@ async def test_templatable__int_with_std_string() -> None:
 
 @pytest.mark.asyncio
 async def test_templatable__string_with_non_string_output_type() -> None:
-    """Static string with non-std::string output_type returns raw string."""
+    """Static string with non-std::string output_type returns stateless lambda."""
     result = await cg.templatable("hello", [], ct.bool_)
 
-    assert isinstance(result, str)
-    assert result == "hello"
+    assert isinstance(result, cg.LambdaExpression)
+    assert result.capture == ""
 
 
 @pytest.mark.asyncio
@@ -684,6 +684,15 @@ async def test_templatable__with_to_exp_callable() -> None:
     assert result == 84
 
 
+@pytest.mark.asyncio
+async def test_templatable__with_to_exp_callable_and_output_type() -> None:
+    """When to_exp is provided with non-string output_type, result is lambda-wrapped."""
+    result = await cg.templatable(42, [], ct.int_, to_exp=lambda x: x * 2)
+
+    assert isinstance(result, cg.LambdaExpression)
+    assert result.capture == ""
+
+
 @pytest.mark.asyncio
 async def test_templatable__with_to_exp_dict() -> None:
     """When to_exp is a dict, value is looked up."""