Built-in metrics and icons carry no privilege over registered ones

Built-in metrics (fps, ms, worstMs, and mb where performance.memory exists) are registered through the same MetricDefinition interface a consumer uses, and built-in glyphs through the same registerIcon. Dogfooding the public interface is the only way to know it suffices.

ui ships only what it can compute without domain knowledge; anything renderer- or domain-specific is contributed by the consumer, either pulled via sample() or pushed via track(). The icon name type is BuiltinIconName | (string & {}), which keeps autocomplete on built-ins while accepting any other name.

Considered Options

• Push-only metrics. Every consumer re-implements the refresh-window accumulator, already duplicated three times in this repository.
• A closed icon record. voxel-map's domain glyphs could not be used on a JollyOption, so ui would have to absorb a cube.