New multiplayerMatch.ts (MultiplayerMatch class) drives one round: syncs
ThemeGuessGame/OpponentView to a round:start payload, relays local
category assignments as round:progress, forwards opponent round:progress
to OpponentView, drives timerHud's countdown (submitting the local color
map as round:submit at expiry), and renders a two-column scoreboard on
round:reveal via computeMatchResult.
ThemeGuessGame gains an optional onCategoryAssigned?(id, hex) constructor
callback, invoked as the last step of applyColor.
timerHud's TimerHudOptions gains an optional onExpire?() callback on the
countdown variant, fired once when endsAt passes.
resultView.ts gains renderMultiplayerResult() for the two-column
you/opponent scoreboard + winner banner (equal overall -> explicit draw).
index.html/style.css: additive #mp-scoreboard/#mp-winner-banner/#mp-you-*/
#mp-opponent-* markup and .mp-* styles; existing single-player result
markup untouched aside from adding id="solo-score-meter-wrap" so it can
be toggled off during a multiplayer reveal.