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>

docs/NETCODE.md

@@ -221,6 +221,23 @@ The Warden's Ration exists to keep this path honest: it is dropped instanced on
 every boss kill, so the filter runs in every real fight rather than only in
 tests.
 
+## Which dungeon you enter is a position, not a request
+
+The hub holds a list of portals, each bound to a dungeon id. Pressing interact
+resolves the player's **server-side** position through `SimWorld.portal_at()`,
+and the resulting `PORTAL_USED` event carries which dungeon that entrance opens.
+
+There is no client message that names a dungeon, and there must not be: one
+would let any client ask for the Proving Grounds' loot rate — ten times the drop
+chance — and bring the results back to the hub. The same reasoning as everywhere
+else here; the difference between "the server checks your request" and "there is
+no request" is the whole model.
+
+`enter_instance` carries the portal list (position plus a dungeon index, via
+`NetCodec.encode_portals`) and the id of the dungeon you have arrived in. The
+client needs the latter to scale the boss's health ceiling the same way the
+server did, or the boss bar would sit near empty for an entire easy fight.
+
 ## Item actions ride the input frame
 
 `InputFrame` carries `BTN_USE`, `BTN_DROP` and a slot byte (10 bytes total, up