Escape custom keys in reserved interpolation regexp

[OskarEichler]

Summary

Escape each reserved interpolation key before composing the cached regular expression.

Custom keys registered with I18n.reserve_key are arbitrary symbols. Treating regex metacharacters in those names as syntax can make unrelated placeholders look reserved.

Verification

• focused external regression model for metacharacter-bearing reserved keys
• 1,607 base-suite runs / 2,674 assertions
• 1,888 Rails-suite runs / 3,020 assertions
• all 46 shipped Ruby files pass syntax checks

[fz-ryanbigg]

But why?

[OskarEichler]

This came from the broad dependency audit rather than a concrete application failure. Regex metacharacters are technically accepted by reserve_key and can alter matching, but I do not have a real-world reserved key that triggers this. I am closing the speculative change.