SMA-10 round 4: pin the Dutch default "de eigen sturing"

The NL default for a hand-back driver without words (handback._BACK_NL) was
changed to "de eigen sturing" in round 2 at the PR #239 review's request
(comment 2495: the article as in the generic clause). The brief does not name
it, so DECISIONS now says where it came from and a test checks the exact Dutch
sentence. No app code change.

Co-Authored-By: Claude <noreply@anthropic.com>

goodwe_controller/DECISIONS/SMA-10-two-idle-states.md

@@ -40,7 +40,9 @@ exists has no driver to release and keeps today's path.
 for shadow mode is built from the roster in `app/handback.py` (EN and NL): a
 roster with no hand-back unit gets None and keeps its old literal, so a GoodWe
 site sees no changed byte. A hand-back unit's words (`Driver.words`: `back`,
-`back_nl`) come from its driver. MQTT `status` is unchanged; the Home battery
+`back_nl`) come from its driver; one without them gets "its own control" /
+"de eigen sturing" (the article as in the generic clause; PR #239 review
+comment 2495, not the brief). MQTT `status` is unchanged; the Home battery
 row of a handed-back unit says "Back under SMA control" and carries
 `handed_back`.