Adds an opt-in world-map layer that marks every rare (rank 4) and rare-elite
(rank 2) creature the client currently has loaded, so a rare that is out shows
as a gold/silver diamond with its name in the current zone. Entities only exist
near the player, so a marker means that rare is genuinely spawned right now.
New RareTrackerLayer follows the existing overlay-layer pattern; the HUD feeds it
the filtered live units each frame. Gated behind a new 'Show Rare Tracker'
Interface setting (persisted), off by default.