[SDK-6f] Extension point: IOutputSink

Expose `IOutputSink` as a public extension point for structured output events (logging, telemetry, external observability). Maps to surface 6 in [RFC #2](https://github.com/ChrisonSimtian/Fallout/issues/98).

## Scope

- Receives structured build / target / tool events.
- Thin wrapper that's easy to register; plugins can *also* register Serilog sinks directly via `builder.Services` (per RFC Q1).
- This is the host-side surface that #104 (AI-friendly output) consumes — confirm shape works for that consumer.

## Cross-ref

- #104 — AI-friendly default console output (consumer surface).