feat: add casual bot backfill policy

server/domain/casual.go

@@ -0,0 +1,59 @@
+package domain
+
+import (
+	"fmt"
+	"time"
+)
+
+type CasualPhase string
+
+const (
+	CasualKickoff CasualPhase = "KICKOFF"
+	CasualLive    CasualPhase = "LIVE"
+)
+
+type CasualSlot struct {
+	Slot     int
+	Team     int
+	PlayerID string
+	IsBot    bool
+}
+
+// BuildCasualLineup freezes the live six-slot shape. Missing humans become
+// explicit server bots; no human is inserted after live play begins by this
+// function.
+func BuildCasualLineup(participants []ConnectParticipant) ([]CasualSlot, error) {
+	if len(participants) < 2 || len(participants) > 6 {
+		return nil, fmt.Errorf("casual lineup needs 2 through 6 humans")
+	}
+	seen := make(map[string]bool, len(participants))
+	teamHuman := map[int]bool{}
+	lineup := make([]CasualSlot, 6)
+	usedSlots := make(map[int]bool)
+	for i, participant := range participants {
+		if participant.PlayerID == "" || participant.Team < 0 || participant.Team > 1 || seen[participant.PlayerID] || usedSlots[i] {
+			return nil, fmt.Errorf("invalid casual participant")
+		}
+		seen[participant.PlayerID] = true
+		usedSlots[i] = true
+		teamHuman[participant.Team] = true
+		lineup[i] = CasualSlot{Slot: i, Team: participant.Team, PlayerID: participant.PlayerID}
+	}
+	if !teamHuman[0] || !teamHuman[1] {
+		return nil, fmt.Errorf("casual lineup requires one human on each team")
+	}
+	for i := range lineup {
+		if lineup[i].PlayerID == "" {
+			lineup[i] = CasualSlot{Slot: i, Team: i % 2, PlayerID: fmt.Sprintf("bot-slot-%d", i), IsBot: true}
+		}
+	}
+	return lineup, nil
+}
+
+func CanCasualBackfill(phase CasualPhase, slot CasualSlot) bool {
+	return phase == CasualKickoff && slot.IsBot
+}
+
+// CasualBackfillPenalty is intentionally zero: a kickoff-only backfill does
+// not receive a hidden-rating update or an abandon/decline cooldown.
+func CasualBackfillPenalty() time.Duration { return 0 }