feat(multiplayer): rotate ranked arenas deterministically

server/cmd/matcher/main.go

@@ -107,7 +107,7 @@ func main() {
 				if err != nil {
 					return domain.PreparedProposal{}, err
 				}
-				return domain.PrepareProposal(id, playlist, formation, participants, domain.DefaultRankedArena(), at)
+				return domain.PrepareProposal(id, playlist, formation, participants, domain.RankedArenaForProposal(id), at)
 			}
 			return domain.PrepareProposal(id, playlist, formation, nil, domain.RankedArena{}, at)
 		},

server/domain/ranked.go

@@ -1,6 +1,9 @@
 package domain
 
-import "fmt"
+import (
+	"crypto/sha256"
+	"fmt"
+)
 
 type RankedParticipant struct {
 	PlayerID   string
@@ -25,6 +28,8 @@ var rankedArenas = map[string]RankedArena{
 	"arena_03": {ID: "arena_03", Path: "res://scenes/arena_03.tscn"},
 }
 
+var rankedArenaOrder = []string{"arena_01", "arena_02", "arena_03"}
+
 // DefaultRankedArena supplies a safe server-owned eligibility decision while
 // the allocator-to-Godot match configuration channel is being completed. A
 // ranked proposal is never admitted based on a mutable command-line boolean.
@@ -32,6 +37,18 @@ func DefaultRankedArena() RankedArena {
 	return rankedArenas["arena_01"]
 }
 
+// RankedArenaForProposal chooses a floor-goal arena deterministically from the
+// proposal identity. The same durable proposal retry therefore cannot change
+// arena, while independent proposals rotate across the registry without
+// mutable worker-local counters.
+func RankedArenaForProposal(proposalID string) RankedArena {
+	if proposalID == "" {
+		return DefaultRankedArena()
+	}
+	digest := sha256.Sum256([]byte(proposalID))
+	return rankedArenas[rankedArenaOrder[int(digest[0])%len(rankedArenaOrder)]]
+}
+
 func ValidateRankedAdmission(participants []RankedParticipant, arena RankedArena) error {
 	if len(participants) != 6 || !validRankedArena(arena) {
 		return fmt.Errorf("ranked admission requirements not met")

server/domain/ranked_test.go

@@ -1,6 +1,9 @@
 package domain
 
-import "testing"
+import (
+	"fmt"
+	"testing"
+)
 
 func rankedParticipants() []RankedParticipant {
 	result := make([]RankedParticipant, 6)
@@ -44,3 +47,21 @@ func TestRankedArenaRegistryExcludesElevatedVariants(t *testing.T) {
 		}
 	}
 }
+
+func TestRankedArenaForProposalIsStableAndRotatesEligibleRegistry(t *testing.T) {
+	first := RankedArenaForProposal("proposal-stable")
+	if first != RankedArenaForProposal("proposal-stable") {
+		t.Fatal("same proposal selected different arenas")
+	}
+	seen := map[string]bool{}
+	for index := 0; index < 128; index++ {
+		arena := RankedArenaForProposal(fmt.Sprintf("proposal-%d", index))
+		if !validRankedArena(arena) {
+			t.Fatalf("proposal selected ineligible arena %+v", arena)
+		}
+		seen[arena.ID] = true
+	}
+	if len(seen) != len(rankedArenaOrder) {
+		t.Fatalf("selected arenas = %v, want every eligible arena", seen)
+	}
+}