Default template's theme forces East-Asian text to fall back to MS Mincho (ＭＳ 明朝)

Summary

Document() (a fresh document created from the bundled default.docx template) configures the
theme and document settings so that East-Asian text resolves to the Japanese font ＭＳ 明朝 (MS Mincho),
even when the user never asked for a Japanese font and even after setting an explicit East-Asian
font with run.font.name. On systems that don't have MS Mincho installed (a standard Chinese/English
Windows), the CJK glyphs are then substituted / rendered in the wrong typeface.

This is distinct from the long-standing API gaps in #346 / #154 / #1396 (which are about
Font.name not writing the w:eastAsia attribute). Those reports are about the user-side setter.
This issue is about the default template itself shipping a mis-configured theme that make the
fallback land on a Japanese font.

Environment

• python-docx: 1.2.0 (verified against master, byte-identical template)
• Word / WPS on a Chinese-locale Windows without the MS Mincho / MS Gothic fonts installed

Reproduction

from docx import Document

doc = Document()          # fresh document from bundled default.docx
p = doc.add_paragraph()
run = p.add_run("中文字体回退测试 Chinese 123")
# Default behavior - we never set any font.
doc.save("repro.docx")

The run python-docx emits

<w:r><w:t>中文字体回退测试 Chinese 123</w:t></w:r>

There is no <w:rPr>/<w:rFonts> on the run at all. So the CJK glyphs must inherit from the theme.

Root cause chain (all confirmed from the shipped template)

The inherited East-Asian font resolves through this chain, and every link is wrong:

1. word/settings.xml — themeFontLang declares the document's East-Asian language as Japanese:

   <w:themeFontLang w:val="en-US" w:eastAsia="ja-JP"/>

2. word/theme/theme1.xml — the minorFont (used by Normal text via docDefaults) has an empty
   East-Asian and complex-script typeface, and its Jpan script font is a Japanese face:

   <a:minorFont>
     <a:latin typeface="Cambria"/>
     <a:ea typeface=""/>
     <a:cs typeface=""/>
     <a:font script="Jpan" typeface="ＭＳ 明朝"/>
     ...
   </a:minorFont>

   (majorFont has the same empty ea/cs with Jpan = ＭＳ ゴシック.)

3. word/styles.xml — docDefaults references the theme East-Asian font instead of a concrete font:

   <w:rFonts w:asciiTheme="minorHAnsi" w:eastAsiaTheme="minorEastAsia" w:hAnsiTheme="minorHAnsi" w:cstheme="minorBidi"/>

4. word/fontTable.xml — the theme declares the Japanese faces ＭＳ 明朝 and ＭＳ ゴシック as
   real fonts (charset 0x80), so they are treated as available.

Resolution: run has no font → docDefaults gives eastAsiaTheme="minorEastAsia" → theme minorFont
East-Asian typeface is empty → because themeFontLang eastAsia is ja-JP, the document's East-Asian
language is Japanese → the Jpan script font (ＭＳ 明朝) is selected.

Even setting run.font.name = '宋体' does NOT fix the CJK glyphs, because python-docx only writes
w:ascii/w:hAnsi (see #346 / #154) and the East-Asian characters still fall through to the theme's
Japanese font.

Expected behavior

• A freshly created Document() should not, by default, resolve CJK text to a Japanese typeface.
• The default template should either declare a neutral/Chinese East-Asian language
  (w:eastAsia="zh-CN"), or specify a concrete, localizable East-Asian font in the theme's
  a:ea/a:cs slots, so CJK glyphs don't silently fall back to MS Mincho.

Possible fix (in the bundled template)

• settings.xml: w:themeFontLang ... w:eastAsia="zh-CN" (or drop the Japanese default).
• theme1.xml: give a:ea/a:cs a non-empty, meaningful typeface (e.g. 宋体/SimSun), or point
  Jpan at a font that shippers commonly have; or set these to a neutral theme-resolvable value.

Related, non-duplicate issues

• feature: Font.name_far_east #154 — feature request for Font.name_far_east (API does not expose East-Asian font setter)
• Cannot set Chinese character font typeface #346 — run.font.name does not apply to Chinese characters (same API limitation)
• Chinese fonts Only the non-Chinese parts are valid #1396 — confirms run.font.name writes only ascii/hAnsi, never eastAsia

These describe the missing eastAsia API; none of them reports that the default template itself
ships a Japanese-language theme that makes the fallback land on MS Mincho.