Create comprehensive documentation of the errors/diagnostics array format
in ExtractionResult and how to integrate assertions in tests.
Closes bf-4cyyf
Acceptance criteria:
- Complete errors array documentation created
- Assertion integration guide written
- Examples provided for common error checking patterns
Verification:
- Documented ExtractionResult.metadata.diagnostics structure
- Provided 8 assertion patterns with code examples
- Included 3 test integration examples (truncated stream, encryption, font glyphs)
- Added helper functions for common assertions
- Listed all diagnostic categories (STRUCT, STREAM, XREF, ENCRYPTION, etc.)
