Plugin telemetry & diagnostics framework

Capacity-permitting follow-on to the v12 plugin SDK.

## Idea

A first-class diagnostics layer for the plugin runtime — emit structured events for plugin load, configure-time errors, contribution registrations, runtime exceptions in plugin-supplied middleware. Surfaces via the existing `IOutputSink` (SDK-6f) so it composes naturally.

## Why not now

Not committed for v12 — track here so the design conversation doesn't get lost. Revisit if/when an early plugin author reports debugging pain that this would address.

## Cross-ref

- v12 SDK milestone #7.