Add a second dungeon: the Proving Grounds, a test harness you walk into

Two labelled portals now stand side by side in the hub. The Proving Grounds
runs the same generator, the same rooms, the same enemies and the same
four-phase Warden -- enemies at a fifth health, the boss at 288 instead of
3600, and trash dropping potions 80% of the time instead of 8%. A manual pass
over loot, the inventory, dropping and every boss phase takes a couple of
minutes rather than a quarter of an hour.

It is multipliers over the shared content rather than a parallel copy: a
duplicated Content would drift the first time anything was tuned, and
"identical but easier" would quietly stop being true. And it is a portal
rather than a launch flag, so the two can be compared back to back without
restarting the server -- which is most of the point.

Which dungeon you enter is resolved from the player's server-side position,
and PORTAL_USED carries the answer. There is deliberately no client message
that names a dungeon: one would let any client ask for the generous loot table
and bring the results back to the hub. Instance matching compares dungeon ids
too, so walking into one entrance can never drop you into the other's run on
timing alone.

SimWorld.portals replaces portal_pos/portal_enabled, enter_instance carries
the portal list and the dungeon id (the client needs the latter to scale the
boss bar's ceiling the way the server scaled the boss), and Protocol.VERSION
goes to 7.

Also pins what happens when two players reach for one item on the same tick:
exactly one gets it -- the loop is sequential and the pickup erases the entity
before the next player looks. The tie-break is join order rather than distance,
which is arbitrary rather than designed, so it is recorded as such.

Stale doc fixed while here: MapGen.build() still claimed the client rebuilds
the map from the seed, which has not been true since map streaming landed and
is the opposite of the rule.

check.sh clean, 288 tests, SMOKE PASS (18 assertions, both dungeon kinds
opened over a real socket), all three diagnostics green.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/sim/sim_world.gd

@@ -33,10 +33,10 @@ var events: Array[Dictionary] = []
 ## unit test build a world without thinking about terrain.
 var map: MapGrid = null
 
-## Set on a lobby world so the interact button can open a dungeon.
-var portal_enabled: bool = false
-## Where the hub's dungeon portal sits. Per-world now that maps vary in size.
-var portal_pos := Vector2.ZERO
+## Dungeon entrances standing in this world. Empty everywhere but the hub.
+## A list rather than a single position because which dungeon an entrance opens
+## is the whole reason there is more than one.
+var portals: Array[SimPortal] = []
 var spawn_point := Vector2(0.0, 240.0)
 ## Arrival protection granted to players entering this world. 0 in the hub,
 ## SimConfig.SPAWN_GRACE_TICKS in a dungeon.
@@ -117,6 +117,20 @@ func spawn_boss(def: BossDef) -> SimBoss:
 	return b
 
 
+## The portal [param at] is standing on, or null. Nearest wins, so two
+## entrances placed close enough to overlap still resolve to one answer instead
+## of to whichever happens to be first in the list.
+func portal_at(at: Vector2) -> SimPortal:
+	var best: SimPortal = null
+	var best_d := SimConfig.PORTAL_RADIUS * SimConfig.PORTAL_RADIUS
+	for portal in portals:
+		var d := at.distance_squared_to(portal.pos)
+		if d <= best_d:
+			best_d = d
+			best = portal
+	return best
+
+
 func alive_player_count() -> int:
 	var n := 0
 	for p in players.values():
@@ -226,9 +240,11 @@ func _step_players() -> void:
 		if edge & InputFrame.BTN_INTERACT:
 			took_item = _try_pickup(p)
 
-		if portal_enabled and not took_item and frame.pressed(InputFrame.BTN_INTERACT):
-			if p.pos.distance_to(portal_pos) <= SimConfig.PORTAL_RADIUS:
-				events.append({"t": SimEvent.Type.PORTAL_USED, "peer": p.peer_id})
+		if not took_item and frame.pressed(InputFrame.BTN_INTERACT):
+			var portal := portal_at(p.pos)
+			if portal != null:
+				events.append({"t": SimEvent.Type.PORTAL_USED, "peer": p.peer_id,
+					"dungeon": String(portal.dungeon)})
 
 
 ## Pull the next input for this player, or coast on the last one. Coasting is