R6(c): share the nested FB builders between notify and gateway transcode

Extract the FlatBuffers builders for the wire tables shared by the backend push
encoder and the gateway edge transcoder — GameView, MoveRecord, StateView,
AccountRef, Invitation and their nested rows — into a new scrabble/pkg/wire
package. Both callers keep their local builder signatures (no call sites move)
but now map their own source types (the backend's notify.* payloads and the
decoded engine.MoveRecord; the gateway's backendclient.* REST DTOs) to neutral
wire.* structs and delegate the construction to package wire, the single
definition of the nested-table layout.

Behaviour-preserving: the verified-identical field sets mean the wire bytes
decode the same, and the notify + transcode round-trip tests pass unchanged. The
fiddly Start/Add/End + reverse-prepend vector boilerplate now lives once; the two
encode files shrink while pkg/wire carries the shared logic.

gateway/internal/transcode/encode_social.go

@@ -5,6 +5,7 @@ import (
 
 	"scrabble/gateway/internal/backendclient"
 	fb "scrabble/pkg/fbs/scrabblefb"
+	"scrabble/pkg/wire"
 )
 
 // Social encoders: friends, blocks, invitations, statistics and GCG. They follow
@@ -12,12 +13,7 @@ import (
 
 // buildAccountRef builds an AccountRef table and returns its offset.
 func buildAccountRef(b *flatbuffers.Builder, r backendclient.AccountRefResp) flatbuffers.UOffsetT {
-	id := b.CreateString(r.AccountID)
-	name := b.CreateString(r.DisplayName)
-	fb.AccountRefStart(b)
-	fb.AccountRefAddAccountId(b, id)
-	fb.AccountRefAddDisplayName(b, name)
-	return fb.AccountRefEnd(b)
+	return wire.BuildAccountRef(b, wire.AccountRef{AccountID: r.AccountID, DisplayName: r.DisplayName})
 }
 
 // buildAccountRefVector builds a [AccountRef] vector using the table-specific
@@ -110,43 +106,28 @@ func encodeStats(r backendclient.StatsResp) []byte {
 
 // buildInvitation builds an Invitation table and returns its offset.
 func buildInvitation(b *flatbuffers.Builder, inv backendclient.InvitationResp) flatbuffers.UOffsetT {
-	inviteeOffs := make([]flatbuffers.UOffsetT, len(inv.Invitees))
+	invitees := make([]wire.InvitationInvitee, len(inv.Invitees))
 	for i, iv := range inv.Invitees {
-		aid := b.CreateString(iv.AccountID)
-		name := b.CreateString(iv.DisplayName)
-		resp := b.CreateString(iv.Response)
-		fb.InvitationInviteeStart(b)
-		fb.InvitationInviteeAddAccountId(b, aid)
-		fb.InvitationInviteeAddDisplayName(b, name)
-		fb.InvitationInviteeAddSeat(b, int32(iv.Seat))
-		fb.InvitationInviteeAddResponse(b, resp)
-		inviteeOffs[i] = fb.InvitationInviteeEnd(b)
+		invitees[i] = wire.InvitationInvitee{
+			AccountID:   iv.AccountID,
+			DisplayName: iv.DisplayName,
+			Seat:        iv.Seat,
+			Response:    iv.Response,
+		}
 	}
-	fb.InvitationStartInviteesVector(b, len(inviteeOffs))
-	for i := len(inviteeOffs) - 1; i >= 0; i-- {
-		b.PrependUOffsetT(inviteeOffs[i])
-	}
-	invitees := b.EndVector(len(inviteeOffs))
-
-	inviter := buildAccountRef(b, inv.Inviter)
-	id := b.CreateString(inv.ID)
-	variant := b.CreateString(inv.Variant)
-	dropout := b.CreateString(inv.DropoutTiles)
-	status := b.CreateString(inv.Status)
-	gameID := b.CreateString(inv.GameID)
-	fb.InvitationStart(b)
-	fb.InvitationAddId(b, id)
-	fb.InvitationAddInviter(b, inviter)
-	fb.InvitationAddInvitees(b, invitees)
-	fb.InvitationAddVariant(b, variant)
-	fb.InvitationAddTurnTimeoutSecs(b, int32(inv.TurnTimeoutSecs))
-	fb.InvitationAddHintsAllowed(b, inv.HintsAllowed)
-	fb.InvitationAddHintsPerPlayer(b, int32(inv.HintsPerPlayer))
-	fb.InvitationAddDropoutTiles(b, dropout)
-	fb.InvitationAddStatus(b, status)
-	fb.InvitationAddGameId(b, gameID)
-	fb.InvitationAddExpiresAtUnix(b, inv.ExpiresAtUnix)
-	return fb.InvitationEnd(b)
+	return wire.BuildInvitation(b, wire.Invitation{
+		ID:              inv.ID,
+		Inviter:         wire.AccountRef{AccountID: inv.Inviter.AccountID, DisplayName: inv.Inviter.DisplayName},
+		Invitees:        invitees,
+		Variant:         inv.Variant,
+		TurnTimeoutSecs: inv.TurnTimeoutSecs,
+		HintsAllowed:    inv.HintsAllowed,
+		HintsPerPlayer:  inv.HintsPerPlayer,
+		DropoutTiles:    inv.DropoutTiles,
+		Status:          inv.Status,
+		GameID:          inv.GameID,
+		ExpiresAtUnix:   inv.ExpiresAtUnix,
+	})
 }
 
 // encodeInvitation builds an Invitation payload.