fix(channelserver): post-RC1 stabilization sprint

Fix rasta_id=0 overwriting NULL in SaveMercenary, which prevented
game state saving for characters without a mercenary (#163).

Also includes:
- CHANGELOG updated with all 10 post-RC1 commits
- Setup wizard fmt.Printf replaced with zap structured logging
- technical-debt.md updated with 6 newly completed items
- Scenario binary format documented (docs/scenario-format.md)
- Tests: alliance nil-guard (#171), handler dispatch table,
  migrations (sorted/SQL/baseline), setup wizard (10 tests),
  protbot protocol sign/entrance/channel (23 tests)

server/channelserver/handlers_mercenary.go

@@ -194,7 +194,12 @@ func handleMsgMhfSaveMercenary(s *Session, p mhfpacket.MHFPacket) {
 	dumpSaveData(s, pkt.MercData, "mercenary")
 	if len(pkt.MercData) >= 4 {
 		temp := byteframe.NewByteFrameFromBytes(pkt.MercData)
-		if err := s.server.charRepo.SaveMercenary(s.charID, pkt.MercData, temp.ReadUint32()); err != nil {
+		rastaID := temp.ReadUint32()
+		if rastaID == 0 {
+			s.logger.Warn("Mercenary save with rasta_id=0, preserving existing value",
+				zap.Uint32("charID", s.charID))
+		}
+		if err := s.server.charRepo.SaveMercenary(s.charID, pkt.MercData, rastaID); err != nil {
 			s.logger.Error("Failed to save mercenary data", zap.Error(err))
 		}
 	}