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/net/net_codec.gd

@@ -424,6 +424,44 @@ static func decode_map_chunks_into(map: MapGrid, data: PackedByteArray) -> int:
 	return applied
 
 
+# --- Portals ----------------------------------------------------------------
+# Sent once, with enter_instance. There are two of them and they never move, so
+# this is about as cold as a message gets -- it is a codec only because the
+# count is variable and RPC arguments are not.
+
+static func encode_portals(portals: Array[SimPortal]) -> PackedByteArray:
+	var b := StreamPeerBuffer.new()
+	b.big_endian = false
+	b.put_u8(mini(portals.size(), 255))
+	for portal in portals:
+		b.put_float(portal.pos.x)
+		b.put_float(portal.pos.y)
+		# By index, like item ids. See Dungeons.ORDER.
+		b.put_u8(Dungeons.index_of(portal.dungeon))
+	return b.data_array
+
+
+## Returns [{ "pos": Vector2, "dungeon": StringName }].
+static func decode_portals(data: PackedByteArray) -> Array[Dictionary]:
+	var out: Array[Dictionary] = []
+	if data.size() < 1:
+		return out
+	var b := StreamPeerBuffer.new()
+	b.big_endian = false
+	b.data_array = data
+	var count := b.get_u8()
+	for _i in count:
+		# 4 + 4 + 1. A truncated packet gives back fewer portals rather than
+		# reading past the end and inventing one at a garbage position.
+		if b.get_available_bytes() < 9:
+			break
+		out.append({
+			"pos": Vector2(b.get_float(), b.get_float()),
+			"dungeon": Dungeons.by_index(b.get_u8()),
+		})
+	return out
+
+
 # --- Character roster -------------------------------------------------------
 # Sent once at login and after any change. Low frequency and carries strings,
 # like the online roster, so it is the same fixed-header-then-utf8 shape.