Honor board_build.f_cpu under the native ESP8266 Arduino toolchain

Dropping it was a real regression: many published ESP8266 configs pin
board_build.f_cpu: 160000000L for timing-sensitive integrations
(MHI-AC-Ctrl documents the 160 MHz requirement in its example), and the
warn-and-drop left those devices at 80 MHz. The option now routes into
CORE.platformio_options under toolchain: arduino for the generator to
consume; other native toolchains keep the warning.

esphome/core/config.py

@@ -593,6 +593,13 @@ async def _add_platformio_options(pio_options: dict[str, str | list[str]]) -> No
                 # platformio/library.py); filters top-level libraries and
                 # discovered dependencies
                 cg.add_platformio_option(key, vals)
+            elif key == "board_build.f_cpu" and CORE.using_toolchain_arduino:
+                # A real-world overclock knob (many published ESP8266 configs
+                # pin 160000000L for timing-sensitive integrations); the
+                # esp8266 native generator reads it for -DF_CPU. Other native
+                # toolchains have no equivalent and fall through to the
+                # warning.
+                cg.add_platformio_option(key, val)
             elif key != "upload_speed":
                 # upload_speed needs no handling: it is read from the raw
                 # config at upload time (upload_using_esptool)

tests/unit_tests/core/test_config.py

@@ -1397,8 +1397,8 @@ def test_esphome_build_internals_are_yaml_only() -> None:
 async def test_add_platformio_options_native_arduino(
     caplog: pytest.LogCaptureFixture,
 ) -> None:
-    """The native ESP8266 Arduino toolchain warns about ignored options the
-    same way the native IDF toolchain does."""
+    """The native ESP8266 Arduino toolchain honors board_build.f_cpu (a
+    real-world overclock knob) and warns about the rest like native IDF."""
     CORE.toolchain = Toolchain.ARDUINO
     CORE.data[KEY_CORE] = {
         KEY_TARGET_PLATFORM: "esp8266",
@@ -1408,11 +1408,16 @@ async def test_add_platformio_options_native_arduino(
     await config._add_platformio_options(
         {
             "board_build.f_cpu": "160000000L",
+            "board_build.filesystem": "littlefs",
             "upload_speed": "115200",
         }
     )
 
-    assert "esphome->platformio_options->board_build.f_cpu is ignored" in caplog.text
+    assert CORE.platformio_options["board_build.f_cpu"] == "160000000L"
+    assert "board_build.f_cpu is ignored" not in caplog.text
+    assert (
+        "esphome->platformio_options->board_build.filesystem is ignored" in caplog.text
+    )
     assert "'arduino' toolchain" in caplog.text
     assert "upload_speed" not in caplog.text