Synced from sheet-skill-spec. Chart labels (plotArea.plot.labels and per-series
labels) are toggled by object existence — passing labels at all turns data
labels on, even when value/category/series/percentage are all false (server
falls back to showing value). Models repeatedly try `{ value: false, category:
false, series: false }` to disable, which silently shows the value fallback.
The reference doc now spells out both directions: pass labels to show, omit
the whole labels field to hide.
Also picks up earlier spec-side drift not yet propagated:
- pivot-table reference: +pivot-list info return + overlap validation
- flag-defs: cell-matrix fan-out cap default 200000 -> 50000 (#1578)
910b0d4c40