Lobby lifecycle: give MatchData a status instead of inferring it from seat occupancy #1328

Server.MatchData (types.ts:408) has no field for "has this match started", so the
lobby infers it from seat occupancy — a match is running once every seat has a name.

That works while every match has a fixed number of seats. It is never true for a table
you can join, so it blocks #1327.

 interface MatchData {
   gameName: string;
   players: { [id: number]: PlayerMetadata };
+  status: 'open' | 'running';   // the lobby's lifecycle, not the game's
+  creator?: PlayerID;           // who may start it
   gameover?: any;
 }

Plus POST /games/:name/:id/start and LobbyClient.startMatch. Fixed-seat matches still
flip to running on their own when the last seat fills, so nothing changes for them.

Two things read the seat count and shouldn't

plugin-player builds G.players from ctx.numPlayers at setup
(plugin-player.ts:86) and short-circuits opponent on ctx.numPlayers === 2
(plugin-player.ts:70).

- if (ctx.numPlayers === 2)
+ if (game.minPlayers === 2 && game.maxPlayers === 2)

What the game declared is stable for the life of the match; the live count is not. The
plugin also needs a hook, or a newly seated player never gets a G.players entry.

The React lobby counts filled seats to decide whether to show Play. It should read
status, and create matches at minPlayers.