Double-click the margin to create and edit a header or footer (#960)

[dkhlapov]

#960: double-click the margin to create and edit a header or footer

Closes #960. In Print Layout, double-clicking the top or bottom margin of a page now creates the header or footer that page shows, if needed, and edits it, as in Word. Different first page and odd/even pages are respected. Before, the double-click only worked when a header already existed.

What changed:

• Layout: each page records which header/footer it shows, even when there is none (header_footer_zone).
• Commands: insert.editHeader / insert.editFooter take an optional {"page"} for agents. This also fixes editing the wrong header on a "Different First Page" page. An out-of-range page is an error.
• Closing: insert.closeHeader removes a newly created header or footer that is still empty, so the docx stays clean.
• On a later section's page: a new header goes into that page's section.

Tests:

• edit_header_on_a_page_creates_its_slot (engine: default, first-page and even-page cases, a clean round-trip, out-of-range pages).
• double_click_margin_creates_header_and_footer (kittest).
• cargo xtask ci passes.

🤖 Generated with Claude Code

[dkhlapov]

Review follow-ups (a12b772):

• Making a header or footer by double-click is now a real edit. It goes through new mutating commands insert.createHeader/insert.createFooter/insert.removeEmptyHeaders, so it's refused in a protected document, undoable and recorded in macros. Going into an existing header stays pure.
• Saving leaves out a new header that's still empty. A header brought back by Undo is cleaned up on the next close.
• A new header on a later section's page goes into the first section, so it links to every section like Word's default.
• Tests: creating_a_header_is_an_undoable_protected_edit, an_empty_new_header_is_never_saved and new_header_links_to_previous_and_even_pages_edit_what_they_show. Full CI passed.

[dpkruse]

Tested on Windows 11 with a letter that uses Different First Page: a logo and contact details in a borderless 3-column table in the first-page header, and a "Page N" default header on later pages.

Before (0.7.0): double-clicking page 1's header put the caret in the default header, so typing never appeared on page 1.
With this PR (head a3dc2e6, built locally): double-clicking page 1's header edits the first-page header. I typed into one of its table cells and the text saved into the first header part. ✅

Still open (maybe out of scope here): Insert › Header › Edit Header on the ribbon still opens the default header when the cursor is on page 1, so nothing seems to happen and the typing goes into a header that isn't shown. Word opens the first-page header in that case. Headless repro on this branch, with any Different First Page document:

wordcraft-cli run --file test.docx --cmd insert.editHeader --cmd 'text.insert={"text":"X"}' --save out.docx

X lands in the w:type="default" header. With insert.editHeader={"page":0} it lands in first, as expected.

Happy to file the ribbon case as a separate issue if that's easier.

[echelon]

Thanks for the contribution, @dkhlapov! This has landed in main. If you haven't already, come join us on Discord: https://discord.gg/artcraft