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/ROADMAP.md

@@ -20,8 +20,8 @@ What "everything passes" currently means. Numbers move; the shape does not.
 | Gate | Covers | Runtime |
 | --- | --- | --- |
 | `tools/check.sh` | every script parses and type-checks | ~5s |
-| `tools/test.sh` | 266 GUT tests, no SceneTree | ~3s |
-| `tools/smoke.sh` | 16 assertions over a real ENet socket: handshake, auth, character creation and persistence, portal, escape, hard kill, polite disconnect | ~40s |
+| `tools/test.sh` | 288 GUT tests, no SceneTree | ~3s |
+| `tools/smoke.sh` | 18 assertions over a real ENet socket: handshake, auth, character creation and persistence, both dungeon kinds, escape, hard kill, polite disconnect | ~40s |
 | `diag_prediction.tscn` | client-prediction gap, with injected clock drift | ~10s |
 | `diag_progression.tscn` | kill → xp → level → health, death → retire → roster, swap guards | ~10s |
 | `diag_loot.tscn` | drop → snapshot → pick up → persist → use → drop, and both loot visibilities on the wire | ~10s |
@@ -70,6 +70,40 @@ draws whatever it holds. The defence is what the server declines to send.
 
 ---
 
+## Dungeon kinds · *done*
+
+Two entrances stand side by side in the hub, labelled, and open different runs.
+
+| Dungeon | `Dungeons` id | Enemy HP | Boss HP | Loot chance |
+| --- | --- | --- | --- | --- |
+| Warden's Descent | `warden_descent` | ×1 | ×1 (3600) | ×1 (trash 8%) |
+| Proving Grounds | `proving_grounds` | ×0.2 | ×0.08 (288) | ×10 (trash 80%, clamped) |
+
+The Proving Grounds is a **test harness you can walk into**: same generator,
+same rooms, same enemies, the same four-phase Warden — everything simply dies
+faster and drops more. A manual pass over loot, the inventory, dropping and all
+four boss phases takes a couple of minutes instead of a quarter of an hour, and
+because it is a portal rather than a launch flag you can compare the two back to
+back without restarting the server.
+
+Adding a third dungeon is one entry in `Dungeons.ORDER` plus one more `P` marker
+in the lobby stamp — the Nth marker, in reading order, opens the Nth entry.
+
+Three things worth knowing:
+
+- **It is multipliers over the shared content, not a copy of it.** A duplicated
+  `Content` would drift the moment anything was tuned, and "identical but
+  easier" would quietly stop being true.
+- **Which dungeon you enter comes from where you stand.** `SimWorld.portal_at()`
+  resolves the server-side position; the `PORTAL_USED` event carries the answer.
+  No client message names a dungeon, which is what stops anyone picking the
+  generous loot table from the real run.
+- **`accepts_new_party_member` takes the dungeon id.** Without that, walking
+  into the Proving Grounds would drop you into whatever standard run happened
+  to still be forming.
+
+---
+
 ## Art and audio · *first pass done*
 
 Placeholders are gone: terrain, actors and bullets are sprites, and four sounds