HsvColorPicker: read the color swatch from the SV handle's child
("Handle/Swatch") instead of a standalone root child; drop the dead
leftover-alpha bar and root-swatch fallback handling.
PaintHudPanel.prefab: restructure the picker so the swatch lives under
the SV handle and wire _swatch accordingly.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>