Returns the highest Sandbox.BaseInventoryItem.Value item we're allowed to switch to, or null if there's nothing switchable. Items flagged Sandbox.BaseInventoryItem.ShouldAvoid (e.g. empty guns) are only picked when nothing better exists. Override for game-specific priority - runs on the host for engine-driven switches (remove, drop, pickup, loadout), so per-player preference data must be host-available.