Inline sink emit calls

[carole-lavillonniere]

Summary

Refactoring addressing suggested improvement #1 from @thrau in this comment #127 (review)

The goal is to make reading the code easier, be more explicit and idiomatic go, and remove some lines of code.

Changes

• Replace package-level EmitXxx(sink, ...) helpers with direct sink.Emit(output.XxxEvent{...}) calls at all sites
• Change Event from a generic union constraint to a sealed marker interface (sealedEvent() unexported method), enabling an exported Sink.Emit(event Event) without generics
• Remove ~88 lines of boilerplate emit helpers; retain SpinnerStart/SpinnerStop/SpinnerStartWithDuration as value constructors (they encode the MinDuration default)

[carole-lavillonniere]

@claude review once