Move MissingKeyHandler, MissingKey, MissingKeyAction, and PluralRule
function types to interface.go for better discoverability.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Rename MissingKeyAction to MissingKey and add OnMissingKey() handler.
Keeps focus on missing keys only for QA - no unnecessary trace overhead.
- MissingKey: dispatched when T()/C() can't find a key in ModeCollect
- OnMissingKey(): register handler for missing key events
- SetActionHandler(): deprecated, use OnMissingKey()
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>