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/Contracts/IShaderCompiler.cs

@@ -144,6 +144,6 @@ public readonly struct ShaderReflectionData
 public interface IShaderCompiler : IDisposable
 {
     Result<ShaderCompileResult> Compile(ref readonly ShaderCompilationConfig config, Allocator allocator);
-    Result<GraphicsCompiledResult> CompilePass(IPassDescriptor descriptor, ref readonly ShaderCompilationConfig additionalConfig, Key64<ShaderVariant> key);
+    Result<GraphicsCompiledResult> CompilePass(ref readonly PassDescriptor descriptor, ref readonly ShaderCompilationConfig additionalConfig, Key64<ShaderVariant> key);
     Result<GraphicsCompiledResult, ErrorStatus> LoadCompiledCache(Key64<ShaderVariant> key);
 }