Add endnote support (word/endnotes.xml) #94

Document.footnotes, Footnotes.add_footnote() and Run.add_footnote_reference() landed in #34, but the endnote half of the same feature did not. A document's endnotes are unreachable: word/endnotes.xml is never loaded, there is no EndnotesPart, and w:endnoteReference in a run is invisible to the API.

The two are near-identical in the schema — both w:footnote and w:endnote are CT_FtnEdn, both parts are CT_Footnotes/CT_Endnotes wrapping a sequence of those, and w:footnoteReference and w:endnoteReference are both CT_FtnEdnRef. ST_FtnEdn (normal, separator, continuationSeparator) is already in oxml/simpletypes.py and its docstring already says it covers w:endnote/@w:type. CT.WML_ENDNOTES and RT.ENDNOTES are already in opc/constants.py.

So this is mostly a matter of generalising what parts/footnotes.py, oxml/footnotes.py and footnotes.py already do rather than writing a new feature from scratch.

Where the prior art is

br-rkdrnf/python-docx, branches footnotes and bering, has a combined implementation under docx/fntent/ (fntent.py, footnoteReference.py, endnoteReference.py) with docx/oxml/fntent.py and docx/parts/fntent.py, plus default-endnotes.xml and default-footnotes.xml templates. It handles footnotes and endnotes through one code path. It is written against upstream v0.8.7 and predates our footnote work, so it is a design reference rather than something to port.

Shape

• EndnotesPart in parts/endnotes.py, alongside FootnotesPart
• CT_Endnotes / reuse of CT_FtnEdn in oxml/footnotes.py
• Document.endnotes -> Endnotes, mirroring Document.footnotes
• Endnotes.add_endnote(), Endnote.text, Endnote.endnote_id
• Run.add_endnote_reference()
• Creating the part on demand when the document has none, the way FootnotesPart does
• Endnote position and numbering live in w:sectPr/w:endnotePr and w:settings/w:endnotePr, both of which are already in the _tag_seq lists — exposing them is optional follow-up, not part of this

Note that Document.iter_inner_content()'s docstring already mentions endnotes; check what it does today and make it consistent.