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>

tests/mvp/test_premium_room_guest_flow.gd

@@ -1,4 +1,5 @@
 extends SceneTree
+const ManaOutage = preload("res://tests/helpers/mana_outage.gd")
 var checks := 0
 var failures := 0
 var runtime: Variant
@@ -59,7 +60,7 @@ func run() -> void:
 	var mid: Dictionary=runtime.create_snapshot()
 	expect(runtime.restore_snapshot(mid), "real guest and in-progress premium service restore together")
 	var elapsed: int=runtime.facility_service.get_service(service_id).elapsed_minutes
-	state.set_resource(&"mana",0)
+	ManaOutage.hold(runtime, state)
 	clock.advance_minutes(15)
 	var paused: ServiceInstance=runtime.facility_service.get_service(service_id)
 	expect(paused.state==ServiceInstance.State.PAUSED and paused.elapsed_minutes==elapsed, "zero mana pauses actual guest lodging without consuming service time")
@@ -70,7 +71,7 @@ func run() -> void:
 	var saved := SaveService.load_snapshot(disk_path)
 	expect(saved[&"ok"] and runtime.restore_snapshot(saved[&"snapshot"]), "restore real guest, paused service and empty mana from disk")
 	SaveService.delete_save(disk_path)
-	state.set_resource(&"mana",100)
+	ManaOutage.release(runtime, state)
 
 	for minute in 370:
 		clock.advance_one_minute()