Add support for up to 6 players.

CLAUDE.md

@@ -6,6 +6,28 @@ Go backend (pure rules engine in `internal/game`, WebSocket rooms in
 
 - `mise run test` — Go tests · `mise run check` — go vet + frontend tsc
 
+## Seats, sides, and packs
+
+Three distinctions are easy to conflate and worth keeping straight:
+
+- **Seat vs. side.** A game seats 2, 4, or 6 players, and each round pairs
+  them off into simultaneous battles (`schedule.go`). Inside a
+  `BattleResult` everything is indexed by *side* — 0 or 1 within that one
+  battle — including `BattleEvent.Seat`/`Target`, `Lineups`, `Survivors`,
+  and `ManaAfter`. `Seats` maps side → seat, and `Side(seat)` maps back.
+  `WinnerSeat` is the deliberate exception: it's a real seat, because it's
+  the only field that means anything outside the battle. When adding a
+  per-side field to a battle, index it by side and say so.
+- **One battle vs. the round.** `resolveBattles` loops the round's pairings;
+  `runBattle(first, second)` resolves one of them and mutates no persistent
+  state. Anything that should happen once per round rather than once per
+  battle (clearing per-round banks, say) belongs in `resolveBattles`, not
+  `finalizeBattle`.
+- **Packs are plural.** `Game.Packs` is a list whose tier decks shuffle
+  together. Anything reading printed card data must go through
+  `TierContentsForPacks` / `CatalogForPacks` / `g.packList()`, never a
+  single pack — a game can hold Trumpets, Mana, and Ailments at once.
+
 ## Keep the AI in sync with game behavior
 
 **Whenever game behavior changes — rules, cards, effects, phases, log