test(multiplayer): add real-Redis integration coverage for the candidate index

Every existing RedisCandidateIndex test runs against miniredis -- a
from-scratch Go reimplementation of the Redis command set, not real
Redis's own float64 score encoding, TTL/expiry, or RESP behavior.
Add an opt-in integration suite (mirroring postgres_integration_test.go's
pattern: //go:build integration, COSMIC_CLASH_REDIS_ADDR-gated) plus
scripts/run_redis_integration.sh against a disposable redis:7-alpine
container, covering:

  - upsert/snapshot/remove against a real server
  - a real TTL actually waited out (not miniredis's manual
    FastForward), proving expiry really happens on the wire
  - the documented 'Redis restart or lost keyspace' repair path
    exercised against an actual FLUSHALL, not a simulated empty map,
    including that the repair actually persists back to Redis (a
    second snapshot reads it without a second durable-source call)

Verified stable across 3 runs with -race against a real container.

scripts/run_redis_integration.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+repo_root="$(cd "$(dirname "$0")/.." && pwd)"
+container_name="cosmic-clash-redis-integration"
+
+cleanup() {
+  docker rm -f "$container_name" >/dev/null 2>&1 || true
+}
+trap cleanup EXIT
+
+cleanup
+docker run --rm -d --name "$container_name" \
+  -p 56379:6379 redis:7-alpine >/dev/null
+
+for attempt in $(seq 1 30); do
+  if docker exec "$container_name" redis-cli ping >/dev/null 2>&1; then
+    break
+  fi
+  if [ "$attempt" = 30 ]; then
+    echo "Redis did not become ready" >&2
+    exit 1
+  fi
+  sleep 1
+done
+
+cd "$repo_root/server"
+COSMIC_CLASH_REDIS_ADDR="127.0.0.1:56379" \
+  go test -tags integration ./store -run TestRealRedis -count=1