Adds playerIndex to FilterInput, backing ShooterIdentity.player_index --
the cluster-assigned shooter, 0-based and stable only within one video.
Deliberately separate from userId, which is populated only once a cluster
has been confirmed against a registered account. Multiplayer sessions are
usually played against someone without a Railbird login, so filtering by
userId cannot express "the other player" in the common case; the cluster
index always can.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>