Pin the board-table equality, note the flash_freq audit

write_project rejects a board missing from either table, so a new test
asserts the two stay exactly equal (the subset check is how d1_wroom_02
went missing). The hardcoded --flash_freq 40 carries the audit note:
all 45 supported boards ship 40 MHz, and a platform bump adding one
that does not must revisit it.

tests/unit_tests/build_gen/test_arduino8266.py

@@ -1208,3 +1208,12 @@ def test_generate_ld_scripts_unreadable_header_forces_regeneration(
         ) as mock_run:
             _run_generate_ld_scripts(paths)
         mock_run.assert_called_once()
+
+
+def test_board_tables_are_equal() -> None:
+    """write_project rejects a board missing from either table, so the two
+    must stay exactly in sync (the build-surgery test only checks the
+    subset direction, which is how d1_wroom_02 went missing)."""
+    from esphome.components.esp8266.boards import BOARDS
+
+    assert set(BOARDS) == set(ESP8266_BOARD_BUILD)