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/repo_character.go

@@ -193,8 +193,15 @@ func (r *CharacterRepository) ReadGuildPostChecked(charID uint32) (time.Time, er
 	return t, err
 }
 
-// SaveMercenary updates savemercenary and rasta_id atomically.
+// SaveMercenary updates savemercenary and optionally rasta_id.
+// When rastaID is 0, only the mercenary blob is saved — the existing rasta_id
+// (typically NULL for characters without a mercenary) is preserved. Writing 0
+// would pollute GetMercenaryLoans queries that match on pact_id.
 func (r *CharacterRepository) SaveMercenary(charID uint32, data []byte, rastaID uint32) error {
+	if rastaID == 0 {
+		_, err := r.db.Exec("UPDATE characters SET savemercenary=$1 WHERE id=$2", data, charID)
+		return err
+	}
 	_, err := r.db.Exec("UPDATE characters SET savemercenary=$1, rasta_id=$2 WHERE id=$3", data, rastaID, charID)
 	return err
 }