Fix core properties namespace conflict

[JackSpiece]

Summary

Importing docxtpl also imports docxcompose when the subdocument extra is installed. docxcompose.utils changes the process-wide python-docx nsmap["cp"] entry to the custom-properties namespace. Later core-property assignments then append an element in that namespace instead of updating the existing core property.

This restores the standard core-properties mapping after the docxcompose imports. docxcompose keeps its own NS mapping, so its custom-property and subdocument behavior is unchanged.

The regression test renders a template, sets keywords, then inspects docProps/core.xml and verifies there is exactly one keyword element in the standard core-properties namespace.

Fixes #471

Verification

• All 37 repository test scripts pass individually
• Full flake8 target passes
• Source distribution and wheel build successfully
• Core-property, custom-property, and subdocument smoke tests pass with docxcompose 1.4.1 and 2.2.0