Add native footnote authoring and lookup - #1616

Add native footnote creation and lookup, with precise reference placement and editable note content.

Document.add_footnote(after=run, text=...) inserts a separate reference run without changing adjacent text or formatting. Document.footnotes exposes ordinary notes, and Run.footnote_ids exposes existing reference IDs. Notes support the existing block-container and run APIs.

The implementation registers a typed footnotes story part, creates separator entries, reserves existing IDs, preserves note styles and numbering settings, and validates body or table-cell reference locations before adding package structures. It does not add endnotes, field-based reuse, note deletion, or pagination controls.

Related to #1 and the broader proposal in #1392. This draft offers a narrower creation and lookup API for discussion. It is based directly on upstream master and contains no fork packaging or other feature commits.

Validation:

• 1,628 upstream unit tests passed.
• 651 acceptance scenarios passed.
• New-module type checks and lint passed.
• Word-authored fixture round trips passed.
• Microsoft Word opened a generated note and confirmed reference placement and multi-paragraph content.