Update API.md table of contents

Added missing sections:
- Helper Functions
- Framework Integration
- Advanced Usage
- Domain Error Mapping (thiserror)

TOC now reflects complete document structure.

Author: blackwell-systems

API.md

@@ -8,6 +8,10 @@ Complete API documentation for error-envelope.
 - [Formatted Constructors](#formatted-constructors)
 - [Custom Errors](#custom-errors)
 - [Builder Pattern](#builder-pattern)
+- [Helper Functions](#helper-functions)
+- [Framework Integration](#framework-integration)
+- [Advanced Usage](#advanced-usage)
+- [Domain Error Mapping (thiserror)](#domain-error-mapping-thiserror)
 
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