fix: preserve GB18030 CLOB write boundaries

[skhe]

Summary

• Keep GB18030 CLOB PUT_DATA chunks on complete character boundaries, preventing incorrect database-side CLOB length near 16 KB boundaries.
• Run ARM Linux real-DM CI across Python 3.9–3.13 and both UTF-8 / GB18030 server encodings, with an explicit UNICODE assertion.
• Document the regression and vendored driver patch.

Validation

• Official ARM DM8 UTF-8: 166 passed, 2 deselected, 0 skipped.
• Official ARM DM8 GB18030: 166 passed, 2 deselected, 0 skipped.
• Vendored Go module: go test ./... passed.
• Pinned CI image locally confirmed CHARSET=0 yields UNICODE=0.