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.DSL/ShaderCompiler/DSLShaderSyntax.cs

@@ -11,7 +11,7 @@ internal struct PropertyDeclaration
     public Token scope;
     public Token type;
     public Token name;
-    public FunctionCallDeclaration? propertyConstructor;
+    public List<Token>? propertyInitializer;
 }
 
 internal struct ValueDeclaration
@@ -44,7 +44,7 @@ internal class PassSyntax
     public HlslDeclaration? hlsl;
     public List<Token>? defines;
     public List<Token>? includes;
-    public List<FunctionCallDeclaration>? keywords;
+    public List<List<Token>>? keywords;
     public List<FunctionCallDeclaration>? functionCalls;
 }