feat: derive authoritative ranked tiers

server/domain/rating.go

@@ -65,6 +65,62 @@ type RankedProfile struct {
 	SeasonHistory []string
 }
 
+type RankTier string
+
+const (
+	RankTierProvisional RankTier = "PROVISIONAL"
+	RankTierBronze      RankTier = "BRONZE"
+	RankTierSilver      RankTier = "SILVER"
+	RankTierGold        RankTier = "GOLD"
+	RankTierPlatinum    RankTier = "PLATINUM"
+	RankTierDiamond     RankTier = "DIAMOND"
+)
+
+// TierBand is backend configuration, not client input. Bands are evaluated in
+// ascending minimum-rating order and the highest matching band wins.
+type TierBand struct {
+	Tier      RankTier
+	MinRating float64
+}
+
+type TierPolicy struct {
+	bands []TierBand
+}
+
+func NewTierPolicy(bands []TierBand) (TierPolicy, error) {
+	if len(bands) == 0 || bands[0].MinRating > 0 {
+		return TierPolicy{}, fmt.Errorf("tier policy must start at or below zero")
+	}
+	copyBands := append([]TierBand(nil), bands...)
+	for i, band := range copyBands {
+		if band.Tier == "" || math.IsNaN(band.MinRating) || math.IsInf(band.MinRating, 0) || (i > 0 && band.MinRating <= copyBands[i-1].MinRating) {
+			return TierPolicy{}, fmt.Errorf("tier bands must have unique ascending finite thresholds")
+		}
+	}
+	return TierPolicy{bands: copyBands}, nil
+}
+
+// RankedTier is the only tier derivation entry point. It deliberately accepts
+// RankedProfile rather than Rating, so a casual rating cannot be accidentally
+// exposed as a ranked tier. The caller serializes this result from the
+// authoritative backend response; clients do not reproduce these thresholds.
+func RankedTier(profile RankedProfile, policy TierPolicy) (RankTier, error) {
+	if profile.RankedGames < 0 || len(policy.bands) == 0 || math.IsNaN(profile.Value) || math.IsInf(profile.Value, 0) {
+		return "", fmt.Errorf("invalid ranked tier input")
+	}
+	if RankedIsProvisional(profile) {
+		return RankTierProvisional, nil
+	}
+	tier := policy.bands[0].Tier
+	for _, band := range policy.bands {
+		if profile.Value < band.MinRating {
+			break
+		}
+		tier = band.Tier
+	}
+	return tier, nil
+}
+
 type RankedSeason struct {
 	SeasonID     string
 	StartsAt     time.Time