[sugi]

Summary

Follow-up to a Copilot review comment on #24: the ERA_DEFS / ERA_NORTH_DEFS arrays were frozen but the Era structs inside were not, so a caller mutating an era returned by Utils.find_era or ERA_BY_NAME could corrupt process-wide era definitions. This freezes the structs at definition time (including the ad-hoc 皇紀/西暦 entries), which covers every lookup path without adding per-call copies on the conversion hot path.

The ERA_JD_LOOKUP builder keeps working as-is since it dups entries before adjusting them (dup of a frozen struct is unfrozen); this also stays correct if #24 (which removes that builder) merges in either order.

Test plan

• TDD: new spec asserting all era definition structs and find_era results are frozen, confirmed failing first
• bundle exec rspec: 66 examples, 0 failures; RuboCop: no offenses

🤖 Generated with Claude Code

https://claude.ai/code/session_01Q1Pf4Mrtd7zB8Lrjc499t9

[gemini-code-assist]

Code Review

This pull request ensures that era definitions and individual Era objects are frozen to prevent unintended modifications. Specifically, it freezes the Era instances assigned to ERA_BY_NAME in lib/wareki/common.rb, freezes each Era object inside the definition arrays in lib/wareki/era_def.rb, and adds a unit test in spec/utils_spec.rb to verify that these definitions are indeed frozen. There are no review comments, and I have no feedback to provide.

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