rename: unit/sub-unit -> subdevice, matching OCF terminology

"Unit"/"sub-unit" from #199's multi-indoor-device support wasn't OCF
idiomatic -- OCF calls each component of a composite device a
"subdevice" (see subdeviceIdList), so rename SubUnit -> Subdevice
throughout: the registry module, coordinator state, BoundEntity's
subdevice field, diagnostics keys (subdevices/subdevices_skipped/
subdevice_probes), entity unique_id prefixes (unit1_/sub_<uuid>_ ->
subdevice1_/subdevice_<uuid>_), device-name fallback labels, golden
fixtures, tests, README, and the adding-device-support skill.

Breaking change to entity unique_ids and diagnostics keys, acceptable
since this hasn't been released yet.

tests/test_entity.py

@@ -17,12 +17,12 @@ class _FakeCoordinator:
     def __init__(self, last_resources):
         self.last_resources = last_resources
 
-    def canonical_resources(self, sub_unit):
+    def canonical_resources(self, subdevice):
         # Every bound entity in this test file uses the default MAIN
-        # sub-unit (identity transform), so the canonical view is just the
+        # subdevice (identity transform), so the canonical view is just the
         # raw snapshot -- same shape as the real
         # LocalThingsCoordinator.canonical_resources for a device with no
-        # sub-units (issue #177).
+        # subdevices (issue #177).
         return self.last_resources