ZATCA Phase-2 Simplified Invoice Integration

I am upgrading an existing ASP.NET Core application to full ZATCA (Fatoora) Phase 2 compliance and now need the simplified-invoice generation piece completed. The scope is strictly invoice generation—tax math and archival are already handled elsewhere.

What I need from you is a clean, well-documented C# service that plugs into my current solution and:

• Constructs a compliant simplified-invoice payload with every mandatory Phase 2 tag, digital signature, unique identifier, and cryptographic stamp.
• Sends the payload to the ZATCA clearance/reporting endpoint, captures the response, and stores the cleared invoice number and hash.
• Returns the base64 QR string so I can place it on the printed receipt.
• Includes a concise README covering certificate setup, sandbox vs. production switching, and an example controller call.

Acceptance criteria: running your code against the ZATCA sandbox with my certificate must generate an approved simplified invoice whose QR code scans correctly in the official app.

Please build with .NET 8 , follow standard dependency-injection patterns, and avoid third-party libraries beyond those expressly permitted by ZATCA unless we discuss them first.