feat(diva): implement Diva Defense (UD) system

Add full Diva Defense / United Defense system: schema, repo layer,
i18n bead names, and RE-verified packet handler implementations.

Schema (0011_diva.sql): diva_beads, diva_beads_assignment,
diva_beads_points, diva_prizes tables; interception_maps/points
columns on guilds and guild_characters.

Seed (DivaDefaults.sql): 26 prize milestones for personal and
guild reward tracks (item_type=26 diva coins).

Repo (DivaRepo): 11 new methods covering bead assignment, point
accumulation, interception point tracking, prize queries, and
cleanup. Mocks wired in test_helpers_test.go.

i18n: Bead struct with EN/JP names for all 18 bead types (IDs
1–25). Session tracks currentBeadIndex (-1 = none assigned).

Packet handlers corrected against mhfo-hd.dll RE findings:
- GetKijuInfo: u8 count, 512-byte desc, color_id+bead_type per entry
- SetKiju: 1-byte ACK; persists bead assignment to DB
- GetUdMyPoint: 8×18-byte entries, no count prefix
- GetUdTotalPointInfo: u8 error + u64[64] + u8[64] + u64 (~585 B)
- GetUdSelectedColorInfo: u8 error + u8[8] = 9 bytes
- GetUdDailyPresentList: correct u16 count format (was wrong hex)
- GetUdNormaPresentList: correct u16 count format (was wrong hex)
- GetUdRankingRewardList: correct u16 count with u32 item_id/qty
- GetRewardSong: 22-byte layout with 0xFFFFFFFF prayer_end sentinel
- AddRewardSongCount: parse implemented (was NOT IMPLEMENTED stub)

server/channelserver/handlers_reward_test.go

@@ -70,13 +70,17 @@ func TestHandleMsgMhfUseRewardSong(t *testing.T) {
 	server := createMockServer()
 	session := createMockSession(1, server)
 
-	defer func() {
-		if r := recover(); r != nil {
-			t.Errorf("handleMsgMhfUseRewardSong panicked: %v", r)
-		}
-	}()
+	pkt := &mhfpacket.MsgMhfUseRewardSong{AckHandle: 12345}
+	handleMsgMhfUseRewardSong(session, pkt)
 
-	handleMsgMhfUseRewardSong(session, nil)
+	select {
+	case p := <-session.sendPackets:
+		if len(p.data) == 0 {
+			t.Error("Response packet should have data")
+		}
+	default:
+		t.Error("No response packet queued")
+	}
 }
 
 func TestHandleMsgMhfAddRewardSongCount(t *testing.T) {
@@ -193,16 +197,16 @@ func TestEmptyHandlers_MiscFiles_Reward(t *testing.T) {
 	server := createMockServer()
 	session := createMockSession(1, server)
 
-	tests := []struct {
+	// Handlers that accept nil and take no action (no AckHandle).
+	nilSafeTests := []struct {
 		name string
 		fn   func()
 	}{
-		{"handleMsgMhfUseRewardSong", func() { handleMsgMhfUseRewardSong(session, nil) }},
 		{"handleMsgMhfAddRewardSongCount", func() { handleMsgMhfAddRewardSongCount(session, nil) }},
 		{"handleMsgMhfAcceptReadReward", func() { handleMsgMhfAcceptReadReward(session, nil) }},
 	}
 
-	for _, tt := range tests {
+	for _, tt := range nilSafeTests {
 		t.Run(tt.name, func(t *testing.T) {
 			defer func() {
 				if r := recover(); r != nil {
@@ -212,4 +216,18 @@ func TestEmptyHandlers_MiscFiles_Reward(t *testing.T) {
 			tt.fn()
 		})
 	}
+
+	// handleMsgMhfUseRewardSong is a real handler (requires a typed packet).
+	t.Run("handleMsgMhfUseRewardSong", func(t *testing.T) {
+		pkt := &mhfpacket.MsgMhfUseRewardSong{AckHandle: 1}
+		handleMsgMhfUseRewardSong(session, pkt)
+		select {
+		case p := <-session.sendPackets:
+			if len(p.data) == 0 {
+				t.Error("handleMsgMhfUseRewardSong: response should have data")
+			}
+		default:
+			t.Error("handleMsgMhfUseRewardSong: no response queued")
+		}
+	})
 }