The instruction a generator hands to an {@see ArtifactBehaviorScaffolder}: write
the one behavior class for a freshly-scaffolded artifact — the Handler for an
Action, the Provider for a Card/Chart/Report/Resource/Tile.
The generator owns the POPO (the artifact class) and the where ({@see $targetDir}
/ {@see $fileName}, both already computed against the app's ui5/ tree). The scaffolder
owns the what — which stub renders into that slot. The default writes Core's plain
marker ({@see $defaultStubPath}); the SDK rebinds the seam and, for {@see ArtifactType::Action},
writes a typed SdkActionHandlerInterface handler instead (spec sdk-action-messaging-v1.0
§Phase-D). The seed variance for Chart/Tile is resolved before this DTO is built —
{@see $defaultStubPath} already points at the seed-selected stub — so the scaffolder never
re-derives it.