Persist header and footer edits on save (original work by @aashish in #42)

[satoryu]

Summary

Makes header and footer edits persist on save/stream. Until now Document#headers and #footers (added in #173 / #174) were read-only — editing the returned Nokogiri documents had no effect because update only wrote back document.xml and styles.xml. This extends update to write each header/footer back into the archive.

doc = Docx::Document.open("example.docx")
doc.headers["header1"].at_xpath("//w:t").content = "New header text"
doc.save("example.docx")   # change is now persisted

Approach

Each header/footer is written back keyed by its original file name (the headers/footers Hash key, e.g. "header1"):

headers.each { |name, content| replace_entry "word/#{name}.xml", content.serialize(save_with: 0) }
footers.each { |name, content| replace_entry "word/#{name}.xml", content.serialize(save_with: 0) }

Using the stored file name (rather than reconstructing header#{index+1}.xml) means documents with multiple or non-sequential header/footer files round-trip correctly without risk of overwriting the wrong entry.

Credit

Based on the original read/write implementation by @aashish in #42. They are credited as a co-author on the commit and should be credited in the release notes when this ships.

Tests

• Adds a round-trip spec: open multi_doc.docx, edit the header and footer text, save, reopen, and assert the changes persisted.
• Full suite green locally (141 examples, 0 failures).

Closes #42

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