A directed span between two bounds of the same form, with per-side closings, a step, and an overflow policy.
An arc always lives inside an {:arcs, scope, grain, arcs} node.
The node owns the scope and the grain — the arc holds only the
bounds, so the two can never disagree. The smart constructors
guarantee that every arc's chains match its node.
On a bare coarse unit the closing covers the whole unit:
until: february, right: :open excludes all of February;
right: :closed includes all of it.
step: nil means "no sampling": the arc covers the whole block
between its bounds. A {1, grain} step is the same set, so the
constructors normalize it to nil — one canonical spelling for
one meaning. The overflow policy comes from the bound points and
acts where a day number lands in a short month.
Summary
Types
@type t() :: %Zocam.Span.Arc{ from: Zocam.Point.chain(), left: Zocam.Intervals.closing(), overflow: Zocam.Point.overflow(), right: Zocam.Intervals.closing(), step: Zocam.Span.step() | nil, until: Zocam.Point.chain() }