Persist material scalars, reset the HUD, and split alpha from the picker

Save and restore the emission intensity and the three camo knobs
(half-Lambert, half-Lambert brightness, specular dampen) with a named
painting. The .paint format moves to v3 with a small scalar block
between the header and the pixel maps; v2 paintings still load with
their painter values left untouched. Loading re-syncs the emission and
camo sliders, and peers pick the values up for free through the
existing owner-authoritative paint NetworkVariables.

Add a HUD reset (Lib_ResetHud button plus context-menu fallback) that
behaves as if the saved hud-layout file were deleted: it removes the
file, restores panels to their authored prefab positions, and resets
the brush values to the painter/material defaults captured before any
carry/restore runs.

Remove the alpha bar from HsvColorPicker so alpha is driven by
standalone Slider_AlbedoAlpha / Slider_EmissionAlpha bindings; the
picker now writes RGB only and the sliders write alpha only, with no
feedback between them. Add a generic SliderCurve component that reshapes
a slider's handle position into a non-linear value (with an inverse for
setting values back), and make the slider wiring curve-aware.

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

Assets/Scripts/SliderCurve.cs.meta

@@ -0,0 +1,2 @@
+fileFormatVersion: 2
+guid: 2e66618c2739718459e4ecd2253c4ba7