Support nested object and array values without crashes

[skhe]

Summary

• Describe object elements in VARRAYs using user-visible type metadata, and reuse the element descriptor for every array position.
• Encode nested array pointers and return errors for malformed complex elements instead of aborting Python.
• Add real-DM regressions for nullable nested objects, object arrays, and nested decimal arrays; test modern SSL CA and hostname verification with local TLS handshakes.

Verification

• GB18030 complete real-DM suite: 191 passed, 6 deselected.
• UTF8 object/array suite: 12 passed.
• Vendored Go tests, Go bridge build, and third-party patch checks passed.

Cross-schema object arrays and other DM8 server versions remain outside the current environment.