ci: weaver live-check gate for SDK self-observability events

[cijothomas]

Wire weaver registry live-check into CI so every SDK-emitted self-observability event declared in an in-tree semconv registry is exercised end-to-end on every PR. Fail the job if a declared event receives zero samples or has event-level violations. SDK-side companion to open-telemetry/semantic-conventions#3723 (otel.sdk.component.shutdown is the first event).

Reference implementations

• otel-arrow MVP: #3047
• otel-arrow refactor onto composite actions: #3175
• Weaver composite actions: open-telemetry/weaver#1448

Sketch

• In-tree registry (e.g. semconv/) with manifest.yaml pulling upstream semconv and one groups/event.<name>.yaml per declared event.
• A runnable that emits the events (dedicated example preferred over reusing examples/basic-otlp).
• Workflow using weaver-live-check-{start,stop} + a registry-driven assert step (zero-sample / violation gate, copied from otel-arrow). fail-on: none until weaver lands per-event coverage (#1474) and severity-scoped exit codes (#1473).
• Not required-status initially; promote later.