0648 — Metric scope belongs to the diagnostic declaration

Supersedes [0647]‘s scope ownership and representation clauses only.

Context

[0647] put the typed MetricScope vocabulary in pkg/metrics while DiagnosticMetric.Scope stored plain text. Building a definition therefore required a cast, and a declaration could carry an unknown scope until a separate conventions test found it.

Decision

• MetricScope and Vulkan’s built-in scope constants live in common/diagnostic beside DiagnosticMetric.
• DiagnosticMetric.Scope and NewDiagnosticMetric use MetricScope. Construction rejects an unknown scope.
• metrics.MetricDefinition.Scope uses diagnostic.MetricScope directly. The future pkg/vulkan surface aliases that declaration like its other reachable exported vocabulary.
• Kind and unit remain strings in the diagnostic declaration because their behavior and validation belong to the metrics domain.

Consequences

• A registered metric cannot carry an invalid scope, and definition conversion needs neither a cast nor a second scope type.
• [0647]‘s registry, catalog ownership, handles, and read semantics remain in force.