feat: roundtrip user-defined object types

[skhe]

Summary

• Resolve user-defined object metadata from ALL_TYPES and ALL_TYPE_ATTRS, including nested object fields.
• Bind and fetch OBJECT values through the Go driver while retaining decimal precision and Unicode text.
• Return explicit errors for invisible or unsupported object types and reject malformed nested object members without a driver panic.
• Add real DM8 regression tests for flat objects, nested objects, and missing types.

Verification

• Local OrbStack DM8: 128 real-database tests passed on Python 3.10.
• Object regression: 3 tests passed on each of Python 3.9, 3.11, 3.12, and 3.13.
• Go bridge and vendored driver tests passed; patch consistency and git diff --check passed.

Scope

This PR covers user-defined CLASS/RECORD objects. Array and BFILE support remain separate work.