Add pipette (eyedropper) brush mode and wire it into the HUD

Pipette samples all painted channel values (albedo, metallic, smoothness,
occlusion, emission) at the clicked texel and adopts them into the brush
with every opacity forced to 1, instead of painting. The screen raycast is
extracted into a shared TryRaycastPaintMesh helper used by painting, preview
and the pipette. A pipette toggle is added to the HUD panel prefab and wired
in PaintHud.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/MeshPainter.Pick.cs.meta

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