feat(xerrors): initial stable release v0.9.0
Structured application errors with typed codes, cause chaining, key-value context fields, and zero-import logz enrichment bridge. What's included: - `*Err` type implementing error, errors.Unwrap, json.Marshaler, ErrorCode(), and ErrorContext() - Twelve typed Code constants aligned with gRPC canonical status names - New / Wrap factory constructors plus InvalidInput / NotFound / Internal convenience constructors - Builder methods WithContext and WithError for attaching structured fields and causes - Duck-typed ErrorCode() / ErrorContext() bridge so logz auto-enriches log records without an import Tested-via: todo-api POC integration Reviewed-against: docs/adr/
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