Upgrade to C++20 and fix all compilation warnings

- Upgrade from C++17 to C++20
- Remove unused helper functions (selectSpawnPreset, parseVec3Csv, parseYawPitchCsv)
- Mark unused parameters with [[maybe_unused]] attribute
- Remove unused variables (nameColor, currentRace, steppingUp, steppingDown, awayFromWallMotion)
- Fix all -Wunused-* warnings

Build now completes with zero warnings.

src/game/world.cpp

@@ -3,11 +3,11 @@
 namespace wowee {
 namespace game {
 
-void World::update(float deltaTime) {
+void World::update([[maybe_unused]] float deltaTime) {
     // TODO: Update world state
 }
 
-void World::loadMap(uint32_t mapId) {
+void World::loadMap([[maybe_unused]] uint32_t mapId) {
     // TODO: Load map data
 }