Add gold diamond markers for every flight master the player has already
discovered (knownTaxiMask_), read from TaxiNodes.dbc and filtered to the
current continent/map being displayed:
- WorldMapTaxiNode struct carries canonical WoW coords + known flag
- WorldMap::setTaxiNodes() accepts the per-frame list from game_screen
- renderImGuiOverlay() projects each known node to UV, draws a gold
diamond (AddQuadFilled) with a dark outline, and shows the node name
as a tooltip on hover
- GameHandler::isKnownTaxiNode(id) checks knownTaxiMask_[] efficiently
- Markers update live — newly discovered nodes appear without reopening
the map
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