P06-01: Implement approved A06 finite Mana reserve (MP-01-r02 recommended)

Initial 100, capacity 300, world tree +20 per 10 game minutes. Each boundary
pays registered upkeep demand before storing surplus; overflow is discarded
and recorded, 0 stops the whole tower. Furnace offers are judged after
generation and demand payment. New saves carry mana_reserve and Operations
rollback includes it; legacy saves keep a 0..300 amount and reject others.

Outage tests that relied on Mana 0 persisting now hold the outage with a
disabled demand sink, since the world tree recovers at the next boundary.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/mana/mana_network_service.gd

@@ -231,6 +231,15 @@ func get_storage_current(node_id: StringName) -> int:
 	return int(_nodes[node_id][&"current"])
 
 
+## Upkeep demand of every registered consumer, including disabled ones.
+func get_total_consumer_demand() -> int:
+	var total := 0
+	for node_id: StringName in _nodes:
+		if int(_nodes[node_id][&"kind"]) == NodeKind.CONSUMER:
+			total += int(_nodes[node_id][&"demand"])
+	return total
+
+
 func get_last_report() -> Dictionary:
 	return _last_report.duplicate(true)