[lvgl] Hoist lazy_once import, document thread-safety

Addresses two stylistic notes on the PR:

- Move `from ..helpers import lazy_once` to module scope in
  widgets/__init__.py. helpers.py only depends on esphome and
  esphome.const, so there's no circular-import risk that would justify
  the function-local import. Keep the `from ..schemas import
  base_update_schema` deferred because schemas.py imports WidgetType
  from this module.
- Document that lazy_once is single-threaded and retries build() on
  exception (no negative-cache), so future callers know the contract
  if the helper migrates to esphome/core/helpers.py.

esphome/components/lvgl/helpers.py

@@ -16,6 +16,11 @@ def lazy_once(build: Callable[[], T]) -> Callable[[], T]:
     Used to defer voluptuous schema construction until first validation. Many
     of the lvgl schemas would otherwise be built at module-import time even for
     YAMLs that never reach them.
+
+    Not thread-safe — concurrent first-callers would each run ``build``. esphome
+    config validation is single-threaded so this is fine in practice. If
+    ``build`` raises, the cache stays empty and the next call retries; there is
+    no negative-cache.
     """
     cached: list[T] = []
 

esphome/components/lvgl/widgets/__init__.py

@@ -48,6 +48,7 @@ from ..defines import (
     join_enums,
     literal,
 )
+from ..helpers import lazy_once
 from ..lv_validation import lv_int
 from ..lvcode import (
     LvConditional,
@@ -80,8 +81,11 @@ def _lazy_update_schema(widget_type: "WidgetType"):
     voluptuous schemas and is invoked once per WidgetType at import time. The
     caller (register_action) only needs a validator, so wrap the build in a
     closure that materialises on the first validation and caches the result.
+
+    The ``from ..schemas import base_update_schema`` import stays inside the
+    closure because ``schemas`` imports ``WidgetType`` from this module — top-
+    level would deadlock the import.
     """
-    from ..helpers import lazy_once
 
     def build():
         from ..schemas import base_update_schema