[core] decouple main loop cadence from scheduler wake timing (#15792)

tests/integration/fixtures/loop_interval_decoupling.yaml

@@ -0,0 +1,60 @@
+esphome:
+  name: loop-interval-decouple
+  on_boot:
+    priority: -100
+    then:
+      - lambda: |-
+          // Raise loop_interval_ to 500ms. With the decoupling fix the
+          // component phase should run ~twice per second while the 50ms
+          // scheduler interval below still fires at its requested cadence.
+          App.set_loop_interval(500);
+      # Start measurement after 1s so boot transients settle.
+      - delay: 1000ms
+      - lambda: |-
+          id(loop_at_start) = id(loop_counter)->get_loop_count();
+          id(sched_at_start) = id(sched_count);
+          ESP_LOGI("test", "MEASUREMENT_STARTED loop=%d sched=%d",
+                   id(loop_at_start), id(sched_at_start));
+      # Observe for 2s.
+      - delay: 2000ms
+      - lambda: |-
+          int loop_delta = id(loop_counter)->get_loop_count() - id(loop_at_start);
+          int sched_delta = id(sched_count) - id(sched_at_start);
+          ESP_LOGI("test", "MEASUREMENT_DONE loop_delta=%d sched_delta=%d",
+                   loop_delta, sched_delta);
+
+host:
+api:
+logger:
+  level: INFO
+  logs:
+    loop_test_component: WARN   # Silence per-loop log spam
+
+external_components:
+  - source:
+      type: local
+      path: EXTERNAL_COMPONENT_PATH
+
+globals:
+  - id: sched_count
+    type: int
+    initial_value: "0"
+  - id: loop_at_start
+    type: int
+    initial_value: "0"
+  - id: sched_at_start
+    type: int
+    initial_value: "0"
+
+loop_test_component:
+  components:
+    - id: loop_counter
+      name: loop_counter
+
+interval:
+  # Fast scheduler interval — with the decoupling fix this should fire at
+  # its requested 50ms cadence regardless of loop_interval_.
+  - interval: 50ms
+    then:
+      - lambda: |-
+          id(sched_count) += 1;