Implements PROTOCOL.md's private-room flow: room:create/room:join
with a collision-checked numeric code, up to a 5-match repeating
series per room via game/session.js, random-theme selection that
excludes the previous match's theme, and room:closed for matchLimit
or quit (rooms never touch matchmaking's ban store). themeMode:
'chosen' gets a narrow resolveChosenTheme() stand-in pending #7's
real theme-voting flow.
Also adds a minimal opponentName lookup to registry.js
(registerSocket's new optional name arg + getPlayerName) since
room:joined needs it, mirroring the same addition landing on #5's
branch.
