Add see-through X-ray reveal overlay for players

Add a second, additive HDR emission color (_XRayColor) that reveals a
player through walls as a pulsing energy glow, driven during matches:

- Shader: XRayOverlay pass (ZTest Greater, additive) for occluded parts
  plus a matching additive term in the forward fragment for visible
  parts, so the glow shows everywhere and never double-blends. Exposes
  _XRayColor, _XRayPulseSpeed and _XRayPulseStrength for a breathing pulse.
- Render it after opaques via a RenderObjects feature (custom XRayReveal
  light-mode tag) on both PC and Mobile renderers, so the through-wall
  test is depth-sort independent and no longer drops out at some camera
  angles/distances.
- MeshPainter drives _XRayColor alpha through a framerate-independent
  fade (xrayFadeDuration) rather than a hard toggle.
- Reveal is driven per avatar on every client: all remaining un-found
  hiders glow during the Resolution phase, and a hider whose
  ConcealmentMeter EmbeddedPercent exceeds MatchManager.embeddedRevealPercent
  glows via an owner-authoritative NetworkVariable.

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

Assets/Scripts/MeshPainter.Setup.cs

@@ -260,4 +260,19 @@ public partial class MeshPainter
         if (_materialInstance && _materialInstance.HasProperty(EmissionColorId))
             _materialInstance.SetColor(EmissionColorId, Color.white * Mathf.Max(0f, emissionIntensity));
     }
+
+    /// <summary>Eases the see-through X-ray glow's alpha toward its on/off target (<see cref="XRayRevealed"/>)
+    /// at a constant rate, so the reveal fades in/out over <c>xrayFadeDuration</c> seconds instead of popping.
+    /// Rgb is left untouched (the authored glow colour). Only writes the material on an actual change, and
+    /// snaps instantly when the duration is 0.</summary>
+    private void TickXRayFade()
+    {
+        if (!_materialInstance || !_materialInstance.HasProperty(XRayColorId)) return;
+        Color c = _materialInstance.GetColor(XRayColorId);
+        float step = xrayFadeDuration > 0f ? Time.deltaTime / xrayFadeDuration : 1f;
+        float a = Mathf.MoveTowards(c.a, _xrayTarget, step);
+        if (a == c.a) return; // already at target -> no material write
+        c.a = a;
+        _materialInstance.SetColor(XRayColorId, c);
+    }
 }