Camo: generalize brightness + add persistent saturation

Move brightness out of the half-lambert diffuse term to a final exposure
multiply on the whole shaded body color (diffuse + GI + specular), and add
a saturation grade (0 = neutral, -1 = grayscale, >0 = boosted) on the same
final stage. Saturation rides as a 4th component on CamoKnobs (Vector3 ->
Vector4) so it inherits all three persistence paths: HUD JSON, network
replication (_camo NetworkVariable), and painting-slot metadata. Wires the
renamed Slider_Brightness and new Slider_Saturation. Bumps HudFileVersion
and PaintMetaVersion with load-time remaps so existing paintings keep their
look (old 1=neutral saturation remaps to 0=neutral).

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

Assets/Shaders/PlayerCamoLit.shader

@@ -12,10 +12,12 @@ Shader "Custom/PlayerCamoLit"
 {
     Properties
     {
-        // --- Camouflage knobs (both 0 => identical to URP/Lit) ---
+        // --- Camouflage knobs (HalfLambert 0, Brightness 0, Saturation 0 => identical to URP/Lit) ---
         [Header(Camouflage)]
         _HalfLambert("Half Lambert (0 none, 0.5 classic, 1 flat)", Range(0.0, 1.0)) = 0.5
-        _HalfLambertBrightness("Half Lambert Brightness Shift", Range(-1.0, 1.0)) = 0.0
+        // Legacy property name; now a GENERAL brightness/exposure on the whole shaded body (see PlayerCamo.hlsl).
+        _HalfLambertBrightness("Brightness (general exposure, 0 = neutral)", Range(-1.0, 1.0)) = 0.0
+        _Saturation("Saturation (0 = neutral, -1 = greyscale, >0 = punchier)", Range(-1.0, 1.0)) = 0.0
         _SpecularDampen("Specular Dampen", Range(0.0, 1.0)) = 0.5
 
         // Specular vs Metallic workflow