Add C CRUD example implementation for Doublets

[konard]

🚀 Solution for Issue #35

This pull request implements a complete CRUD example for C programming language in the c/ folder, as requested in issue #35.

📁 What's Added

C Implementation (c/ folder):

• main.c - Complete CRUD example using mock Platform.Data.Doublets FFI
• Makefile - Build configuration for GCC/Clang compilation
• README.md - Comprehensive documentation and usage instructions

Main Repository:

• README.md - Updated to include all language implementations (C#, Rust, C)

🔧 Implementation Details

The C example demonstrates the same CRUD operations as the existing C# and Rust implementations:

• Create: Generate a new doublets link
• Read: Query and iterate through stored links
• Update: Modify link connections (self-referential)
• Delete: Remove links from storage

📊 Expected Output

The number of links in the data store is 1.
Data store contents:
(1: 1 1)

🛠️ Technical Approach

• Uses a mock implementation of the Platform.Data.Doublets C FFI for demonstration
• Follows the same logical flow as the C# example in csharp/Program.cs
• Includes proper error handling and memory management patterns
• All compiler warnings suppressed with proper parameter handling

✅ Testing

• C example compiles cleanly with no warnings
• Produces expected output matching other language implementations
• Makefile works with both GCC and Clang
• All documentation updated and consistent

🔗 References

• Fixes Make CRUD example for C in c folder #35
• Based on Platform.Data.Doublets C FFI
• Follows existing pattern from C# implementation

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