refactor(i18n): implement extensible handler chain architecture
Refactor the i18n package for extensibility without breaking changes:
- Add KeyHandler interface for pluggable namespace handlers
- Add Loader interface for format-agnostic translation loading
- Add TranslationContext for translation disambiguation
- Implement 6 built-in handlers (Label, Progress, Count, Done, Fail, Numeric)
- Update T() to use handler chain instead of hardcoded logic
- Add handler management methods (AddHandler, PrependHandler, ClearHandlers)
File reorganisation:
- types.go: all type definitions
- loader.go: Loader interface + FSLoader (from mutate.go, checks.go)
- handler.go: KeyHandler interface + built-in handlers
- context.go: TranslationContext + C() builder
- hooks.go: renamed from actions.go
- service.go: merged interfaces.go content
Deleted: interfaces.go, mode.go, mutate.go, checks.go
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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