0674 — Playground examples share handle, instance, and lifecycle patterns

Context

Topic reuse, handler placement, lifecycle setup, and variable roles varied between playground examples without a scenario-specific reason.

Decision

• Create each topic handle once and reuse it. Retain registration results only when an operation needs their fields, such as schedule registration.
• Name handles for their domain and instances for their activity. Qualify instance names when several of the same kind exist.
• Always declare consumer handles separately from their registered instances.
• Put consumer handlers in named functions below run.
• Use LifecycleContext with deferred stop in every example and observe cancellation during simulated waits.
• Use routines and routinesCtx for errgroup orchestration.

Consequences

The examples teach one structure while retaining their different scenarios and topic-registration prerequisites. Handler placement and explicit consumer handles are the current choices; the user may revisit them later. CONVENTIONS.md holds the binding rules. The finite examples now respond to shutdown signals, and the metrics handler’s simulated wait is cancellable.