Add Simplified Chinese language support

[JuneQQQ]

Summary

Adds Simplified Chinese support — the whole game (event log, roles, agent
speech, CLI) can now run in English or Chinese. Requested by the project owner.

deepwolf simulate --players 7 --seed 1 --lang zh

Changes

• New deepwolf/i18n.py — a Translator with an en/zh message catalogue
  covering every game event, role name, role summary and faction label. It is a
  leaf module (imports nothing else from deepwolf).
• engine.py — all player-facing event text now flows through the Translator.
  English output is byte-for-byte unchanged (the reproducibility tests still
  pass without modification).
• GameConfig gains a validated lang field; PlayerView carries lang.
• prompts/templates.py — bilingual LLM prompts (system message, asks, format
  instructions). The [[ACTION ...]] trailer now carries lang= so the offline
  MockProvider answers in the right language; JSON reply keys stay English.
• RandomAgent and MockProvider emit localised statements.
• cli.py — a --lang {en,zh} flag on simulate and play; localised outcome
  panel.
• Adds README.zh-CN.md and links it from the English README.

Checklist

• ruff check . passes
• mypy deepwolf passes
• pytest passes (60 tests; 9 new for i18n)
• English output unchanged — existing reproducibility tests untouched
• CHANGELOG.md and README.md updated

Notes for reviewers

The design keeps i18n out of the hot path: the engine holds one Translator
and English strings are identical to before, so no existing game, transcript or
seed changes behaviour. Adding a third language is just another column in the
i18n.py catalogue.

[JuneQQQ]

Code review (post-merge record).

• The decisive design choice: the engine holds one Translator and English message strings are byte-identical to the previous inline f-strings. Verified — every reproducibility test passed without modification, so no existing game, seed or transcript changed behaviour. ✅
• i18n.py is a true leaf module (no deepwolf imports), so any layer can use it and there is no import-cycle risk. ✅
• Player ids (P0) are kept verbatim in both languages — only prose is translated, so machine-readable references stay stable. ✅
• The [[ACTION ... lang=]] trailer is the right channel: it keeps the offline mock language-aware without coupling it to prompt prose, and JSON reply keys stay English so parsing is unchanged. ✅
• Missing translations fall back to English (and unknown keys to the key itself) — graceful, and a good test covers it. ✅
• CI green on 3.10–3.12.

Adding a third language is now just another column in the catalogue. Merged via squash.