createSliderCore

Description#

Pure factory for a slider's value derivation - the brain shared by CngxSlider and each CngxSliderThumb of a range slider. It owns no DI and no DOM: hand it the source signals, get back the clamped value, the track fraction, the aria-valuetext string, and the four mutation helpers the keyboard / pointer handlers call. Snapping and bound-clamping live in CngxSliderCore.setValue, so every write path stays valid without an effect syncing state (Pillar 1).