fix(GUI): DrawSlider drag latch + occlusion/scissor checks

Card 8fba9b.

• FILE_ActiveSliderID latch (FILE_ActiveScrollBar pattern): drag starts on press in-box, persists while left is held with pointer X clamped to the track, releases on mouse-up — knob no longer drops when the pointer leaves the box
• Optional ID param (position-derived fallback key) so sibling sliders can't steal each other's latch
• Optional OnTop param + active-scissor check: covered/clipped sliders can't start drags or fire value changes
• ItemSpawnerCW threads its OnTop through drawRateSlider; QuantumPrediction passes an explicit ID