Fix header/footer dropped after save/stream when parts are stored out of order

[gokulavasan-io]

Changes

Document#update (runs on every #save / #stream) rewrites each header/footer part, but it targets the output file by array position — word/header#{index + 1}.xml — while fetch_headers/fetch_footers read the parts via @zip.glob, which returns entries in the zip's central-directory order (not guaranteed numeric). When a document stores its parts out of numeric order, each part is written to the wrong file; the part that document.xml references as the default header ends up holding a different (often empty) part, so the header/footer disappears in the rendered/converted output.

The fix captures each part's real filename at fetch time and writes each part back to that filename instead of a positional index. It is a no-op for normally-ordered documents and only corrects the out-of-order case.

Lib

File	Change
lib/docx/document.rb	fetch_headers/fetch_footers: capture @header_names/@footer_names (glob order) and read parts by name. #update: write each header/footer back to @header_names[i]/@footer_names[i] instead of word/header#{i+1}.xml.

Tests

File	Change
spec/docx/document_spec.rb	Streaming regression test: builds a docx with header2.xml stored before header1.xml, streams it, asserts header2.xml still holds its own content. Red without the fix, green with it.

Architecture / Flow

A .docx is a zip; headers are separate files. document.xml points at one as the "default".

  glob('word/header*.xml')  →  [header2, header3, header1]   (zip order, NOT numeric)
  fetch_headers array:          idx0      idx1      idx2
                                content   empty     empty

BEFORE (write by index → wrong file):
  idx0 → header1.xml  ← header2's CONTENT
  idx1 → header2.xml  ← header3's EMPTY     ← document.xml's default → now blank ✗
  idx2 → header3.xml  ← header1's EMPTY

AFTER (write by captured name → same file it came from):
  idx0 → header2.xml  ← content   ✓
  idx1 → header3.xml  ← empty     ✓
  idx2 → header1.xml  ← empty     ✓

Impact

Any caller that saves/streams a docx with section-differentiated headers/footers (different-first-page / odd-even) stored out of numeric order — e.g. offer-letter generation, which calls replace_placeholders → #stream → #update. Reproduced end-to-end on a real offer template: header blank before the fix, fully rendered after.

Safety

• Normally-ordered docs: @header_names[i] == "word/header#{i+1}.xml" → byte-identical output (no-op).
• Only the out-of-order case changes — corrupt → correct. Cannot regress a working document.
• Full gem suite passes (137 examples).

Before:

After: