feat(multiplayer): rotate ranked arenas deterministically

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")