v2.1.0 — Japanese template parity and CI smoke checks

Japanese template polish

• Removed parenthesized English names from the Japanese sample slides where the
  Japanese terms are sufficient on their own.
• Kept Japanese body emphasis aligned with the English template by preserving
  the shared \boldcolor{...} emphasis macro.
• Matched the Japanese closing slide's top-spacing logic to the English
  template (\aauheaderheight + 20pt), so both templates use the same layout
  calibration.
• Bibliography content and the References frame remain in English.

CI smoke checks

• Added a structure-parity check for the Japanese and English templates:
  sections, frames, column/block environments, overlay commands, frame labels,
  and closing-slide spacing must stay aligned.
• Added PDF output validation after both templates build: generated PDFs must
  exist, have the same page count, and share the same page size.
• Added render validation with Poppler (pdftoppm) so every generated PDF page
  must be rasterizable in CI.
• Uploads the built PDFs, LaTeX logs, and rendered page PNGs as workflow
  artifacts for pull-request review.
• Updated the official checkout/upload/download artifact actions to current
  Node 24-compatible major versions.
• Pinned the macOS build runner to macos-15 for more reproducible TeX builds.

Release workflow

• Release creation is split into its own tag-only job with write permissions,
  while normal build checks run with read-only repository permissions.
• Tag builds now sync release notes from .github/release-notes/<tag>.md when
  a matching file exists, then attach slide-ja.pdf and slide-en.pdf.

Compatibility

No breaking changes are expected for existing decks. This is a minor release
because it improves template parity and release/CI validation without changing
the public LaTeX macro API.

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