[core] Lazy-load zeroconf/writer/yaml_util in __main__

Every \`esphome\` CLI invocation pays the cost of whatever \`esphome/__main__.py\`
imports at module scope before the requested command even runs. This
moves three heavy imports into the functions that actually use them:

- \`esphome.zeroconf\` (discover_mdns_devices): only needed for the
  name_add_mac_suffix OTA discovery path.
- \`esphome.writer\`: only needed by compile/clean paths.
- \`esphome.yaml_util\`: only needed by codegen, config dump, and rename.

Local measurement drops from ~75ms to ~47ms (-37%) for a cold
\`python -c 'import esphome.__main__'\`. The zeroconf chain alone accounts
for most of the gain — the case that motivated the CI budget check.

Also adds a module-level note explaining the intent so future PRs don't
innocently promote these back to the top.

Retargets four test patches from \`esphome.__main__.discover_mdns_devices\`
to \`esphome.zeroconf.discover_mdns_devices\` now that the symbol is only
bound inside the function that calls it.

esphome/__main__.py

@@ -21,7 +21,7 @@ import argcomplete
 # Note: Do not import modules from esphome.components here, as this would
 # cause them to be loaded before external components are processed, resulting
 # in the built-in version being used instead of the external component one.
-from esphome import const, writer, yaml_util
+from esphome import const
 import esphome.codegen as cg
 from esphome.config import iter_component_configs, read_config, strip_default_ids
 from esphome.const import (
@@ -72,7 +72,12 @@ from esphome.util import (
     run_external_process,
     safe_print,
 )
-from esphome.zeroconf import discover_mdns_devices
+
+# Keep expensive imports (zeroconf, writer, yaml_util, etc.) out of this
+# module's top level. Every `esphome` invocation — including fast paths
+# like `esphome version` — pays the cost of what's imported here before
+# any command runs. Import inside the function that needs it instead.
+# `script/check_import_time.py` enforces a budget in CI.
 
 _LOGGER = logging.getLogger(__name__)
 
@@ -241,6 +246,8 @@ def _discover_mac_suffix_devices() -> list[str] | None:
     """
     if not (has_name_add_mac_suffix() and has_mdns() and has_non_ip_address()):
         return None
+    from esphome.zeroconf import discover_mdns_devices
+
     _LOGGER.info("Discovering devices...")
     if not (discovered := discover_mdns_devices(CORE.name)):
         _LOGGER.warning(
@@ -661,6 +668,8 @@ def run_miniterm(config: ConfigType, port: str, args) -> int:
 
 
 def wrap_to_code(name, comp):
+    from esphome import yaml_util
+
     coro = coroutine(comp.to_code)
 
     @functools.wraps(comp.to_code)
@@ -680,6 +689,8 @@ def wrap_to_code(name, comp):
 
 
 def write_cpp(config: ConfigType, native_idf: bool = False) -> int:
+    from esphome import writer
+
     if not get_bool_env(ENV_NOGITIGNORE):
         writer.write_gitignore()
 
@@ -702,6 +713,8 @@ def generate_cpp_contents(config: ConfigType) -> None:
 
 
 def write_cpp_file(native_idf: bool = False) -> int:
+    from esphome import writer
+
     code_s = indent(CORE.cpp_main_section)
     writer.write_cpp(code_s)
 
@@ -1180,6 +1193,8 @@ def command_wizard(args: ArgsProtocol) -> int | None:
 
 
 def command_config(args: ArgsProtocol, config: ConfigType) -> int | None:
+    from esphome import yaml_util
+
     if not CORE.verbose:
         config = strip_default_ids(config)
     output = yaml_util.dump(config, args.show_secrets)
@@ -1321,6 +1336,8 @@ def command_clean_mqtt(args: ArgsProtocol, config: ConfigType) -> int | None:
 
 
 def command_clean_all(args: ArgsProtocol) -> int | None:
+    from esphome import writer
+
     try:
         writer.clean_all(args.configuration)
     except OSError as err:
@@ -1336,6 +1353,8 @@ def command_version(args: ArgsProtocol) -> int | None:
 
 
 def command_clean(args: ArgsProtocol, config: ConfigType) -> int | None:
+    from esphome import writer
+
     try:
         writer.clean_build()
     except OSError as err:
@@ -1538,6 +1557,8 @@ def command_analyze_memory(args: ArgsProtocol, config: ConfigType) -> int:
 
 
 def command_rename(args: ArgsProtocol, config: ConfigType) -> int | None:
+    from esphome import yaml_util
+
     new_name = args.name
     for c in new_name:
         if c not in ALLOWED_NAME_CHARS:

tests/unit_tests/test_main.py

@@ -2605,7 +2605,7 @@ def test_choose_upload_log_host_discovers_mac_suffix_devices(tmp_path: Path) ->
     }
     with (
         patch(
-            "esphome.__main__.discover_mdns_devices", return_value=discovered
+            "esphome.zeroconf.discover_mdns_devices", return_value=discovered
         ) as mock_discover,
         patch(
             "esphome.__main__.choose_prompt", return_value="mydevice-abc123.local"
@@ -2653,7 +2653,7 @@ def test_choose_upload_log_host_mac_suffix_no_devices_found(
     )
 
     with (
-        patch("esphome.__main__.discover_mdns_devices", return_value={}),
+        patch("esphome.zeroconf.discover_mdns_devices", return_value={}),
         caplog.at_level(logging.WARNING, logger="esphome.__main__"),
         pytest.raises(EsphomeError),
     ):
@@ -2686,7 +2686,7 @@ def test_choose_upload_log_host_default_ota_discovers_mac_suffix(
         "mydevice-def456.local": ["10.0.0.2"],
     }
     with patch(
-        "esphome.__main__.discover_mdns_devices", return_value=discovered
+        "esphome.zeroconf.discover_mdns_devices", return_value=discovered
     ) as mock_discover:
         result = choose_upload_log_host(
             default="OTA",
@@ -2715,7 +2715,7 @@ def test_choose_upload_log_host_default_ota_no_suffix_discovery(
         name="mydevice",
     )
 
-    with patch("esphome.__main__.discover_mdns_devices") as mock_discover:
+    with patch("esphome.zeroconf.discover_mdns_devices") as mock_discover:
         result = choose_upload_log_host(
             default="OTA",
             check_default=None,