Map of array-valued top-level keys to their item signature. Tracks both id-bearing items and anonymous (no-id) items separately because ECharts resolves them differently during merge.
Length of option.media array (responsive).
Sorted list of object-valued top-level keys (e.g. 'grid', 'legend').
Length of option.options array (timeline).
Sorted list of scalar-valued top-level keys (e.g. 'backgroundColor').
Structural signature of an ECharts option object. Only top-level keys are inspected, deep diffing is intentionally avoided for performance and to keep the diff semantics predictable.