v2.1.1 — Typography and inline arrow spacing fixes

Slide typography

• Restored the heavier look of slide titles, frame titles, and emphasized text
  by pinning Rounded-X Mgen+ 1cp to its medium face for regular text and
  black face for bold text.
• Kept the existing Rounded-X Mgen+ 1cp font family and Beamer font settings,
  while making the selected weights deterministic across environments.
• Explicitly maps italic and bold-italic requests back to the matching upright
  rounded-gothic faces, avoiding accidental fallback to thinner faces.

Inline arrows

• Fixed missing spacing before inline arrow macros in Japanese text.
• Updated \arrow, \whitearrow, and \arrowdown to reuse xeCJK's
  \CJKecglue, so arrow spacing follows \xeCJKsetup{CJKecglue=...} and
  \CJKsetecglue.
• Updated the Japanese and English sample slides to use the spacing-aware arrow
  macro directly.

README screenshots

• Regenerated README screenshots from the updated PDFs at 300 dpi.
• Refreshed the Japanese title slide, English title slide, and English sample
  slide images under .github/readme-assets/.

Compatibility

This is a patch release. Existing macro names are unchanged; the arrow macros
now include their own inline spacing so Japanese and English text render
consistently.

Prebuilt PDFs (slide-ja.pdf / slide-en.pdf) are attached by CI.