R4: push enrichment — events carry a state delta, kill the last poll

Enrich the in-app live stream into a delta channel so the UI renders a move from the event without a follow-up game.state, and make the matchmaking poll a stream-down fallback.

- pkg/fbs: trailing fields on opponent_moved (move+game+bag_len), your_turn (move_count), match_found (state), game_over (game), notify (account/invitation/state), MoveResult (rack+bag_len); regenerate Go + TS.
- backend: notify owns the FB encoding (encode.go + payload.go input structs); game/lobby/social map their domain types in. emitMove builds the move delta; game.Service.InitialState feeds match_found/game_started the recipient's initial StateView; friends/invitations notify carry their account/invitation. The move-commit response (submit_play/pass/exchange/resign) returns the actor's refilled rack + bag size.
- gateway: MoveResult transcode carries rack+bag_len.
- ui: pure lib/gamedelta.ts reducer advances the per-game cache keyed on move_count (idempotent + gap-safe); app.svelte seeds the cache on match_found/game_started; Game.svelte applies the delta (commit/pass/exchange/resign drop their load()); NewGame polls only while app.streamAlive is false.
- docs: ARCHITECTURE §10, FUNCTIONAL(+ru), backend/gateway/ui READMEs; PRERELEASE R4 marked done + Refinements.

backend/internal/game/eventwire.go

@@ -0,0 +1,79 @@
+package game
+
+import (
+	"scrabble/backend/internal/engine"
+	"scrabble/backend/internal/notify"
+)
+
+// The mappers below project the game domain into the wire-agnostic notify.* input
+// structs the enriched live events carry (R4). They keep the wire schema out of the
+// game package: notify owns the FlatBuffers encoding, this file only resolves the
+// values (seat display names, last-activity sort key) into its input shapes.
+
+// gameSummary projects a game.Game into the notify.GameSummary embedded in enriched
+// events. names is the seat-indexed display-name slice from seatNames; LastActivityUnix
+// mirrors the gateway view (the current turn's start while active, the finish time once
+// finished).
+func gameSummary(g Game, names []string) notify.GameSummary {
+	seats := make([]notify.SeatStanding, 0, len(g.Seats))
+	for _, s := range g.Seats {
+		name := ""
+		if s.Seat >= 0 && s.Seat < len(names) {
+			name = names[s.Seat]
+		}
+		seats = append(seats, notify.SeatStanding{
+			Seat:        s.Seat,
+			AccountID:   s.AccountID.String(),
+			DisplayName: name,
+			Score:       s.Score,
+			HintsUsed:   s.HintsUsed,
+			IsWinner:    s.IsWinner,
+		})
+	}
+	last := g.TurnStartedAt
+	if g.FinishedAt != nil {
+		last = *g.FinishedAt
+	}
+	return notify.GameSummary{
+		ID:               g.ID.String(),
+		Variant:          g.Variant.String(),
+		DictVersion:      g.DictVersion,
+		Status:           g.Status,
+		Players:          g.Players,
+		ToMove:           g.ToMove,
+		TurnTimeoutSecs:  int(g.TurnTimeout.Seconds()),
+		MoveCount:        g.MoveCount,
+		EndReason:        g.EndReason,
+		Seats:            seats,
+		LastActivityUnix: last.Unix(),
+	}
+}
+
+// playerState projects a StateView into the notify.PlayerState carried by the
+// match_found / game_started events. The rack is re-encoded to wire alphabet indices;
+// the variant alphabet display table is embedded when includeAlphabet is set (an
+// initial view whose recipient may not have cached the variant yet).
+func playerState(v StateView, names []string, includeAlphabet bool) (notify.PlayerState, error) {
+	rack, err := engine.EncodeRack(v.Game.Variant, v.Rack)
+	if err != nil {
+		return notify.PlayerState{}, err
+	}
+	ps := notify.PlayerState{
+		Game:           gameSummary(v.Game, names),
+		Seat:           v.Seat,
+		Rack:           rack,
+		BagLen:         v.BagLen,
+		HintsRemaining: v.HintsRemaining,
+	}
+	if includeAlphabet {
+		tab, err := engine.AlphabetTable(v.Game.Variant)
+		if err != nil {
+			return notify.PlayerState{}, err
+		}
+		ps.Alphabet = make([]notify.AlphabetLetter, len(tab))
+		for i, e := range tab {
+			ps.Alphabet[i] = notify.AlphabetLetter{Index: int(e.Index), Letter: e.Letter, Value: e.Value}
+		}
+	}
+	return ps, nil
+}