feat(glossary): add Japanese (ja) translation glossary

[mmcky]

Adds a Japanese built-in glossary (glossary/ja.json, 357 terms), mirroring the English term set and context fields of the existing zh-cn / fa glossaries so coverage is identical across languages.

What's included

• glossary/ja.json — 357 terms (economics, macro/micro, finance, linear algebra, probability/statistics, time series, network science, economist & mathematician names).
• dist-action/glossary/ja.json — the bundled copy produced by npm run build, so the action loads it automatically when target-language: ja (the CI dist-action freshness guard passes).
• Docs updated: Japanese row in docs/user/glossary.md and the "Current Glossaries" entry in glossary/README.md (🚧 Planned → 🟡 Draft).

Conventions followed

• Same English keys + contexts as the other glossaries (verified programmatically — identical order, no missing/extra terms).
• Abbreviations expand to the full Japanese term with the abbreviation in parentheses, e.g. GDP → "国内総生産（GDP）", OLS → "最小二乗法（OLS）".
• Personal names transliterated to katakana, e.g. Robert Solow → "ロバート・ソロー".

⚠️ Review note

This is an initial draft produced for native-speaker review (per the glossary quality guidelines), marked 🟡 Draft in glossary/README.md. A Japanese-speaking economist should confirm terminology before promotion to ✅ Complete. Two areas to prioritise:

• CJK-origin researcher names are currently katakana placeholders (e.g. Hiroki Kondo → ヒロキ・コンドウ, Yoshi Fujiwara → ヨシ・フジワラ, and the Chinese-origin names). The authors should supply preferred kanji.
• A few term choices worth a second look: 連結状態 (communicating states), 残余費用関数 (cost-to-go function), レイクモデル (lake model).

Note: this PR and #68 (French) both add a row to the same glossary table; whichever merges second will need a trivial rebase.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds a new built-in Japanese translation glossary to the action-translation repository, alongside documentation updates intended to expose target-language: ja support and ensure the bundled dist-action/ artifact contains the new glossary.

Changes:

• Added a new Japanese glossary file (glossary/ja.json) and bundled copy (dist-action/glossary/ja.json) with 357 terms.
• Updated glossary documentation to list Japanese as a draft glossary and reflect current status/metadata.
• Updated user docs to include Japanese in the built-in glossary table and add a draft status note.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
glossary/README.md	Documents Japanese glossary status/contents and updates “Last updated” metadata.
glossary/ja.json	Adds the new built-in Japanese glossary (357 terms).
docs/user/glossary.md	Adds Japanese to the built-in glossaries table and notes draft status.
dist-action/glossary/ja.json	Bundled (built) copy of the Japanese glossary for runtime use in the GitHub Action distribution.