Refactor shader system: arrays, keywords, property syntax

Major refactor of shader compiler and related systems:
- Switch ShaderDescriptor/PassDescriptor to arrays; remove IPassDescriptor
- Rewrite keywords block parser/semantic analysis for flexible syntax
- Change property initializers to brace syntax `{ ... }`
- Simplify TokenStream API (remove ref index params)
- Make GetBindlessIndex return uint (~0u for not found)
- Update shader compilation and variant logic for new descriptors
- Update test shader syntax to match new property/keyword formats
- Add AGENTS.md agent development guide
- Add Antlr4 dependency to Ghost.DSL
- Miscellaneous code style and error handling improvements

Ghost.Graphics/D3D12/D3D12ResourceDatabase.cs

@@ -244,14 +244,14 @@ internal class D3D12ResourceDatabase : IResourceDatabase
         return r.Value.desc;
     }
 
-    public Result<uint, ErrorStatus> GetBindlessIndex(Handle<GPUResource> handle)
+    public uint GetBindlessIndex(Handle<GPUResource> handle)
     {
         ObjectDisposedException.ThrowIf(_disposed, this);
 
         ref var info = ref GetResourceRecord(handle, out var exist);
         if (!exist || !info.Allocated)
         {
-            return ErrorStatus.NotFound;
+            return ~0u;
         }
 
         return (uint)info.viewGroup.srv.Value;