fix(derive): keep components incremental

Let #[slhx::component] preserve inline modules when generated symbols are absent, while still enforcing handler completeness when slhx.syms exists.

req: component/005

req: test/002

slhx-derive/tests/surface.rs

@@ -1,4 +1,9 @@
-use slhx_derive::surface;
+use slhx_derive::{component, surface};
+
+#[component]
+mod local_component {
+    pub const EXISTING: u8 = 1;
+}
 
 #[surface]
 mod ui {
@@ -9,3 +14,8 @@ mod ui {
 fn surface_macro_preserves_inline_module_with_generated_file() {
     assert_eq!(ui::EXISTING, 1);
 }
+
+#[test]
+fn component_macro_preserves_inline_module_without_generated_symbols() {
+    assert_eq!(local_component::EXISTING, 1);
+}