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/transport/test_runtime_transport.gd

@@ -1,4 +1,5 @@
 extends SceneTree
+const ManaOutage = preload("res://tests/helpers/mana_outage.gd")
 
 const FixtureCleanup := preload("res://tests/helpers/ui_audio_fixture_cleanup.gd")
 var _finishing := false
@@ -128,7 +129,7 @@ func _verify_mid_ride() -> bool:
 	_expect(_runtime.restore_snapshot(snapshot), "rewind depletion drill to original passenger state")
 	for cell: Vector3i in shaft[&"stops"]:
 		_runtime.path_service.configure_cell_traffic(cell, 4, 0, true)
-	_state.set_resource(&"mana", 0)
+	ManaOutage.hold(_runtime, _state)
 	_clock.advance_minutes(1)
 	for actor_id: StringName in riders:
 		_expect(_runtime.elevators.get_actor_trip(actor_id).get(&"state") == PathService.ELEVATOR_REQUEST_BOARDED, "no safe landing retains boarded passenger instead of teleporting")
@@ -140,7 +141,7 @@ func _verify_mid_ride() -> bool:
 	_expect(_runtime.restore_snapshot(held_snapshot) and _runtime.create_snapshot() == held_snapshot, "no-landing outage preserves full Runtime save and passenger ownership")
 	for cell: Vector3i in shaft[&"stops"]:
 		_runtime.path_service.configure_cell_traffic(cell)
-	_state.set_resource(&"mana", 100)
+	ManaOutage.release(_runtime, _state)
 	_clock.advance_minutes(45)
 	for actor_id: StringName in riders:
 		_expect(_runtime.elevators.get_actor_trip(actor_id).is_empty() and _runtime.path_service.get_active_elevator_request(actor_id).is_empty(), "landing repair and power restoration release held trip ownership")