Split a range by a step or period, and Values(step) on the discrete types #100

Python portion has iterate(interval, step); Rust rangemap has StepLite; NodaTime's DateInterval enumerates days; Guava's ContiguousSet enumerates a discrete domain. (portion: https://raw.githubusercontent.com/AlexandreDecan/portion/master/README.md ; NodaTime: https://nodatime.org/3.2.x/api/NodaTime.DateInterval.html .) The library has Values() on the discrete types (one element per step of the domain) and nothing for a step of the consumer's choosing, nor for cutting a range into sub-ranges: the billing months inside a contract period, the 30-minute slots inside opening hours, the ISO weeks of a quarter.

Proposed API (per type, since T arithmetic is not generic):

• DateRange.Split(DateRangeStep step) / NodaTime LocalDateRange.Split(Period) → IEnumerable<TRange>: consecutive sub-ranges of the step length, the last one clipped to the range's end.
• DateTimeRange.Split(TimeSpan), DateTimeOffsetRange.Split(TimeSpan), TimeRange.Split(TimeSpan), InstantRange.Split(Duration); Int32Range.Split(int), Int64Range.Split(long), DecimalRange.Split(decimal).
• Values(step) overloads for the discrete types, the stepping form of Values().

Unbounded ranges throw at the call, as Values() does. A calendar-aligned split (first of month, Monday) is where this meets #48 (calendar factories); the two should share the week rule.

Translation: none. generate_series returns rows, not a scalar, which is the documented reason Values() is in-memory only (E31). Say so in docs/efcore.md#what-runs-where.

Done when: Split is documented in docs/ranges.md with the clipping rule, held by claims, and a metamorphic test holds RangeSet.From(range.Split(step)) == RangeSet.From([range]) and that pieces are pairwise adjacent.

Found by the comparable-library survey (portion, rangemap, NodaTime), 2026-10-17.