phase(N.5) Task 1 fixup: remove unused _gl field + IsAvailable

Code quality review caught three related issues:
- _gl field stored but never used (TreatWarningsAsErrors=true would
  catch this on a clean build, but better to fix it before it bites)
- GL constructor parameter became unused after dropping _gl
- IsAvailable => true is misleading: TryCreate's out parameter is
  the canonical signal, the property carries no information

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/Wb/BindlessSupport.cs

@@ -5,20 +5,15 @@ namespace AcDream.App.Rendering.Wb;
 
 /// <summary>
 /// Thin wrapper around <see cref="ArbBindlessTexture"/> + capability detection
-/// for the modern rendering path. Constructed once at startup. Throws if the
-/// extension isn't available — callers must check <see cref="IsAvailable"/>
-/// before constructing for production use.
+/// for the modern rendering path. Constructed once at startup via
+/// <see cref="TryCreate"/>, which returns false if the extension isn't present.
 /// </summary>
 public sealed class BindlessSupport
 {
-    private readonly GL _gl;
     private readonly ArbBindlessTexture _ext;
 
-    public bool IsAvailable => true;  // Construction succeeded
-
-    public BindlessSupport(GL gl, ArbBindlessTexture extension)
+    private BindlessSupport(ArbBindlessTexture extension)
     {
-        _gl = gl;
         _ext = extension;
     }
 
@@ -26,7 +21,7 @@ public sealed class BindlessSupport
     {
         if (gl.TryGetExtension<ArbBindlessTexture>(out var ext))
         {
-            support = new BindlessSupport(gl, ext);
+            support = new BindlessSupport(ext);
             return true;
         }
         support = null;