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/RHI/IResourceDatabase.cs

@@ -59,8 +59,8 @@ public interface IResourceDatabase : IDisposable
     /// Retrieves the bindless index associated with the specified GPU resource handle.
     /// </summary>
     /// <param name="handle">A handle to the GPU resource for which to obtain the bindless index. Must reference a valid, currently registered resource.</param>
-    /// <returns>The bindless index corresponding to the specified GPU resource handle. -1 if the resource does not support bindless access or is not found.</returns>
-    Result<uint, ErrorStatus> GetBindlessIndex(Handle<GPUResource> handle);
+    /// <returns>The bindless index corresponding to the specified GPU resource handle. ~0 if the resource does not support bindless access or is not found.</returns>
+    uint GetBindlessIndex(Handle<GPUResource> handle);
 
     /// <summary>
     /// Retrieves the name of the GPU resource associated with the specified handle.