Instantiate valid DOCX packages from DOTX templates - #1

This is a focused follow-up to your upstream python-openxml#1537 proposal.

The existing loader accepts DOTX packages but retains the template main content type when saving to a .docx path. This follow-up makes Document(template) instantiate a document and save a genuine DOCX package. Lower-level Package.open() keeps the original package type.

It also replaces optional, missing-fixture tests with committed Word-authored templates and coverage for paths, streams, content-type detection, macro-enabled format rejection, source immutability, and preservation of styles, numbering, theme, sections, headers, footers, images, and relationships. Documentation defines the distinction between instantiation and template authoring.

Validation:

• 1,617 unit tests and 651 acceptance scenarios passed on this branch.
• Changed-file lint passed.
• Word opened generated output as format 12 (DOCX), with header, image, and generated text intact.

The branch is based directly on your PR head and includes only this follow-up commit. No fork packaging or other features are included.