Refresh design-backlog docs after Pot's gain/theta cleanup

Drop the now-stale "config.json.templ/.sim still carry dead
Model.kn/Plant.kn keys" note (they were deleted), note that Pot now
always starts at theta_amb (no separate initial-temp param) and that
dropping the gain (efficiency) factor entirely is a partial realism
regression, not just a fix. Point the config-validation item at the
gain key that's still dead in config.json.sim. Also update demo_sud.py
to match the Pot params Pot.__init__ actually reads.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/demos/sud/demo_sud.py

@@ -34,12 +34,10 @@ if __name__ == '__main__':
 	}
 
 	plant_params = {
-		"theta"		: 20,
 		"C"			: 4190,
 		"M"			: 20,
 		"L"			: 0.2,
-		"Td"		: 30,
-		"gain"		: 1.0
+		"Td"		: 30
 	}
 
 	sud = Sud("sude/sud_0010.json")