Add CtePlanViewer, CteResultViewer, and PhaseSummaryViewer to render the
structured v2 payloads (schema_version: 2) the harness now emits for
artifact-gate widgets, per contracts.md. Extract PreviewGrid from
ResultsPanel as a reusable AG Grid component shared by CteResultViewer.
ArtifactView dispatches to the new viewers on a schema_version/shape
guard, falling back to the existing legacy renderers unchanged for
older sessions and replay fixtures.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>