UX-08 2c: short refusals, error texts and option texts

Validator refusals name the setting by its label and key, say what is
wrong and what to change, and keep the rejected value and limit. Option
names and descriptions are short in en and nl; citations moved to DOCS
"For installers" and the test_translations citation check now pins the
instruction on the tab and the record in DOCS. No rule, key, schema,
default or exit code changed. Allowlist 167 -> 43 (log lines only).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LyR88rXETvwhTgx6yHE39p

goodwe_controller/app/arbitrage.py

@@ -262,10 +262,8 @@ def threshold_eur_kwh(opts: dict) -> float:
     v = float(price_option(opts, "cycle_cost_threshold_eur"))
     if not math.isfinite(v) or v < 0:
         raise ValueError(
-            f"cycle_cost_threshold_eur is {v!r}. It is what a round trip COSTS, "
-            f"in EUR/kWh, so it is non-negative and finite - a negative "
-            f"threshold is one every spread clears, which is arbitrage with the "
-            f"brake removed.")
+            f"“Battery cycle cost” (cycle_cost_threshold_eur) is {v!r}. Set "
+            f"it to 0 or more, in EUR/kWh.")
     return v