[scheduler] Force-inline process_to_add() fast path

process_to_add() has the same pattern as cleanup_(): a one-line
inline fast path (check if to_add_ is empty) with the slow path
already out-of-line in process_to_add_slow_path_(). GCC inlines it
on ESP8266 but not on ESP32, emitting a 20-byte out-of-line body.

Use ESPHOME_ALWAYS_INLINE to guarantee inlining on all platforms.
Verified on ESP32 (xtensa-esp32):
- process_to_add() symbol eliminated from binary
- Scheduler::call() idle path: 1 out-of-line call (millis_64 only)

esphome/core/scheduler.h

@@ -138,7 +138,7 @@ class Scheduler {
   // (single-threaded). This is safe because the main loop is the only thread
   // that reads to_add_ without holding lock_; other threads may read it only
   // while holding the mutex (e.g. cancel_item_locked_).
-  inline void HOT process_to_add() {
+  inline void ESPHOME_ALWAYS_INLINE HOT process_to_add() {
     if (this->to_add_empty_())
       return;
     this->process_to_add_slow_path_();