v1.20.0 ✅

• Structured Error Hierarchy — 5 built-in domain errors (NotFoundError, ValidationError, ConflictError, UnauthorizedError, ForbiddenError) with HTTP status code tags and CRTP fluent chaining
• ResultToIResult: Domain Error-Aware HTTP Mapping — reads HttpStatusCode tag for accurate status codes (was always 400)
• Test Coverage Hardening — 150 new tests covering OkIf/FailIf, Try, Combine, Tap, LINQ Task extensions
• Internal Quality — cached computed properties, ExceptionError namespace fix, Result\<T> constructor encapsulation, ToString() override, dead code cleanup, convention-based SmartEndpoints route prefix