[core] Combine set_component_source_ + register_component_ into one call

Add an optional `source_index` parameter to `App.register_component_()`
so codegen can emit a single registration line per component instead of
two. On a typical large config (~120 components) this saves ~120 lines
in generated main.cpp and one virtual-method dispatch per component
during setup().

esphome/core/application.h

@@ -382,7 +382,11 @@ class Application {
 
   /// Register a component, detecting loop() override at compile time.
   /// Uses HasLoopOverride<T> which handles ambiguous &T::loop from multiple inheritance.
-  template<typename T> void register_component_(T *comp) {
+  /// Optionally sets the component source index in the same call to avoid emitting
+  /// a separate set_component_source_() line in generated code.
+  template<typename T> void register_component_(T *comp, uint8_t source_index = 0) {
+    if (source_index != 0)
+      comp->set_component_source_(source_index);
     this->register_component_impl_(comp, HasLoopOverride<T>::value);
   }
 

esphome/cpp_helpers.py

@@ -197,9 +197,9 @@ async def register_component(var, config):
         )
     if name is not None:
         idx = register_component_source(name)
-        add(var.set_component_source_(idx))
-
-    add(App.register_component_(var))
+        add(App.register_component_(var, idx))
+    else:
+        add(App.register_component_(var))
 
     # Collect C++ type for compile-time looping component count
     comp_entries = CORE.data.setdefault("looping_component_entries", [])