v0.1.5 - Tests, error messages, README status

Addresses the remaining items from upstream's original 'Project status' TODO list:

• Tests: xmlrpc request/response round-trip (string, int, bool, struct, array, nested), specifically including that unknown enum values pass through correctly and that malformed/truncated XML returns an error instead of hanging (the exact class of bug fixed in v0.1.3). Plus a few representative convert_gen.go converter tests. Not exhaustive across all 74 generated files, but covers what has actually broken in this fork's history.
• CI: go test -short ./... added to the regeneration workflow, so these actually run continuously.
• Fixed client_test.go's pre-existing TestAuthentication, which required a live server on localhost:40080 and would hang/fail under any CI — now skips under -short.
• Better error messages: Error.Error() no longer prints blank fields for error codes that don't carry an object type/UUID.
• README: rewrote the 'Project status' section against this fork's actual state (2 of upstream's 5 TODO items were already resolved, one was already true from the start) and documented that this fork is actively maintained.