Adds an N-tool for the "no data yet" tile that sits in the exact
footprint where a real chart would go. Default 320×200 matches the
line/bar chart default footprint; dashed border + slate-50 fill +
slate icon signal "chart slot, currently empty". Caller can hint
at the widget type via icon ("line-chart" / "pie-chart" / default
"bar-chart-2").
Intentionally separate from add_empty_state_v0 — that tool is for
inbox/onboarding/no-results full-page empties (has optional CTA,
no dashed border). add_empty_chart_v0 reads as "chart widget is
live, just lacks data yet" rather than "nothing to show on this
screen at all".
Wired through all 3 paths + parity/contract/design-prompt tests.
Handler test covers 6 cases: defaults + dashed stroke + icon
override + size clamping + title/subtitle override + bogus
parent_id rejection.
97343c1d74