Implement ANTLR AST transformation with exact position mapping in links store

[konard]

Summary

This PR implements ANTLR-based AST transformation functionality that transforms code into its Abstract Syntax Tree (AST) inside the links store, with each AST node mapped to its exact position in the source code text.

Key Features Implemented

• Complete Position Mapping: Each AST node includes exact character positions, line numbers, and column positions in the source code
• Links Store Integration: AST nodes are stored using Platform.Data.Doublets with hierarchical relationships preserved
• C# Language Support: Basic C# constructs (classes, methods, properties, statements) are recognized and parsed
• Comprehensive API: Full integration with existing FileStorage class

Files Added/Modified

• New: csharp/Storage/AST/AstNode.cs - Core AST node representation with position mapping
• New: csharp/Storage/AST/CSharpAstTransformer.cs - ANTLR-based C# code transformation
• New: csharp/Storage.Tests/ - Complete test suite with 6 passing tests
• Modified: csharp/Storage/LocalStorage/FileStorage.cs - Added AST transformation methods
• Modified: csharp/Storage/Storage.csproj - Added ANTLR4.Runtime.Standard dependency

API Usage

using var storage = new FileStorage("data.db");

// Transform C# code into AST and store in links
var rootNodeLink = storage.TransformCodeToAst(csharpCode);

// Retrieve all AST nodes from links store  
var allNodes = storage.GetAllAstNodes();

// Get detailed position info for any node
var nodeInfo = storage.GetAstNodeInfo(nodeLink);

Links Store Structure

AST nodes are stored as: AST_NODE_MARKER -> (NODE_TYPE -> (TEXT -> POSITION))

• Position data includes: StartPos, EndPos, StartLine, StartCol, EndLine, EndCol
• Parent-child relationships preserved through additional links

Test Plan

✅ All 6 tests passing:

• AST transformation and links store integration
• Exact position mapping verification
• Parent-child relationship handling
• Error handling for invalid inputs
• Node retrieval and information extraction

dotnet test csharp/Storage.Tests/
# Test Run Successful: 6/6 tests passed

🤖 Generated with Claude Code

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Resolves #77