fix(redis): harden sentinel primary validation

[im0x0ing]

This change hardens Redis startup against Sentinel split-brain.

• Read all Sentinel instances and require a majority before accepting a master.
• Verify the selected candidate really reports role:master via INFO replication.
• Fail closed when existing Redis data is present but no trusted primary can be established.
• Keep the default-ordinal fallback only for first bootstrap on an empty data directory.

Testing

• bash -n addons/redis/scripts/redis-start.sh
• git diff --check
• Test cluster reproduction: normal bootstrap, 1:2 Sentinel split, and fail-closed behavior when no quorum exists.