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/actors/dungeons/dungeon_def.gd

@@ -0,0 +1,45 @@
+class_name DungeonDef
+extends Resource
+## A kind of dungeon run. Same generator, same rooms, same enemies -- what
+## differs is how tough its contents are and how freely they drop.
+##
+## Deliberately a set of multipliers over the shared content rather than a
+## parallel copy of it: a second dungeon that duplicated `Content` would drift
+## from the first the moment anything was tuned, and then "identical but
+## easier" would quietly stop being true.
+
+@export var id: StringName = &"dungeon"
+@export var display_name: String = "Dungeon"
+## One line for the portal label, so a player standing in the hub can tell the
+## entrances apart without reading a wiki.
+@export var subtitle: String = ""
+@export var enemy_hp_mult: float = 1.0
+@export var boss_hp_mult: float = 1.0
+## Multiplies every loot chance, clamped at certain. Guaranteed drops stay
+## guaranteed; there is nothing above 1.0 to reach for.
+@export var loot_chance_mult: float = 1.0
+## Portal colour, and the tint of the dungeon's name on the HUD.
+@export var tint := Color(0.5, 0.9, 1.0)
+
+
+## Scale a freshly built [EnemyDef] in place.
+##
+## Safe to mutate because every caller of `Content.enemy()` gets a new object --
+## the content functions construct one per call. If that ever changes, this has
+## to duplicate first, or one easy dungeon would nerf every hard one.
+func apply_to_enemy(def: EnemyDef) -> EnemyDef:
+	def.max_hp = maxi(1, roundi(float(def.max_hp) * enemy_hp_mult))
+	_scale_loot(def.loot)
+	return def
+
+
+func apply_to_boss(def: BossDef) -> BossDef:
+	def.max_hp = maxi(1, roundi(float(def.max_hp) * boss_hp_mult))
+	_scale_loot(def.loot)
+	return def
+
+
+func _scale_loot(table: Array[LootDrop]) -> void:
+	for entry in table:
+		if entry != null:
+			entry.chance = clampf(entry.chance * loot_chance_mult, 0.0, 1.0)

src/actors/dungeons/dungeon_def.gd.uid

@@ -0,0 +1 @@
+uid://cqert4nr662vl