SAFETY-97: point test_safety97 at MUTATION.md; record the DC-entity option

test_bytecode requires every suite to name MUTATION.md (CI run 1158).
DECISIONS: why the integration's DC battery power entity is not the fix.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019DBUfvndgHwaV339Fvd2gr

goodwe_controller/DECISIONS/SAFETY-97-idle-offset.md

@@ -34,6 +34,25 @@ B sat on the band's edge and never latched:
   start is never quiet, so never learned, so never caught.
 - **Until learned the idle reading is 0 W**: the rule as it was.
 
+## Not chosen: the GoodWe LAN integration's DC battery power
+
+Both inverters also appear through HA's GoodWe integration (B:
+`sensor.kelder_dyness_battery_power`, 0 W at idle; found read-only on the
+live HA 2026-10-04). Pointing `batt_entity` at it removes B's offset by
+configuration, but:
+
+- the band must be safe without it: other installs may not have it;
+- it is polled over UDP on the inverter's Wi-Fi, a second channel that can
+  drop while the board keeps reading (HW-01 lost both once); a missing
+  reading stops the site;
+- SAFETY-37's 35 s bound is derived from the board's 10 s publish; the
+  integration's poll interval (believed 10 s, not checked here) and latency
+  are not measured;
+- DC power is not 0x0518, which SAFETY-30/31/42 were measured against.
+
+It stays an install choice to test on the rig, not a code change here. With
+it, the learned idle reading is simply ~0 W.
+
 ## Where it applies
 
 Every test of "gives nothing / gives / takes" now uses `reading - idle`: