Adds low-risk import-and-assert tests across migrated plugins, guarding the
classes of bugs found while testing this branch:
- is_timeseries charts must expose a datetime control (calendar, horizon, rose,
partition). Partition genuinely lacked one, so add sections.legacyTimeseriesTime
to its control panel (same 500 as paired-t-test otherwise).
- deck.gl Multiple Layers must issue an empty query (self-fetches its layers).
- deck.gl Arc is timeseries only when time_grain_sqla is set.
- Time Pivot's numeric x-axis format must render a number, not the literal
SMART_NUMBER string.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
