Update tkm2re units metadata explicitly

[huangzesen]

Summary

• explicitly update dl.data_att.units after the existing str_element block ensures the nested field exists
• keep the existing str_element structure but avoid relying on nested write-through behavior for existing units fields

Fixes #411

Validation

• static check: rg -n 'dl\.data_att\.units = label|str_element,dl\.data_att' idl/general/misc/tkm2re.pro
• no IDL 9.0/Mac ARM64 runtime was available here, so I could not reproduce the platform-specific failure locally

Review

• Zhipu/GLM review gate: reviewed before PR; no blocking findings; comment wording adjusted per review