Document prebuilt binaries and setup-graphcal

[shunichironomura]

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Summary

Step 7 of the binary distribution plan: document the prebuilt binaries shipped since v0.0.1-alpha.35 and the setup-graphcal v1 action.

• docs/{en,ja}/installation.md, reorganized around what a user wants to do:
  • Install with the installer script (macOS / Linux, Windows PowerShell) comes first.
  • Install with a package manager groups cargo binstall (prebuilt) and cargo install (builds from crates.io, needs Rust 1.95+), both using graphcal@<version> while Graphcal is a pre-release.
  • Build from source keeps only the prerequisite list and the build command.
  • Use in GitHub Actions shows a minimal graphcal-lang/setup-graphcal@v1 workflow (graphcal check + graphcal eval), pinned to 0.0.1-alpha.35.
  • Implementation details are removed: install paths, how the engine is embedded, offline builds, and the commit suffix of --version.
  • The Japanese edition was proofread with the yomiyasu skill.
• README.md: the Quickstart now uses the shell installer and links to the installation guide, instead of saying Rust is required.

Decisions:

• Heading IDs changed (#requirements, #install-from-cratesio, #build-from-a-source-checkout are gone); no incoming links reference them.
• The GitHub Actions example pins version because the language still changes between alpha releases. The example version must be bumped manually when it becomes too old.
• Japanese and English code fences are byte-identical and rendered heading IDs match (checked in the just docs-build output).

AI assistance

• AI used: yes — Claude Code wrote the documentation changes and this description.
• Human review of AI-assisted code: pending

Breaking changes

• No breaking changes
• Breaking changes; the breaking change label is applied