Shift and Expand on ranges (move by a delta, buffer both sides) #101

Intervals.NET offers Shift and Expand through its domain packages (https://www.nuget.org/packages/Intervals.NET); Guava's ContiguousSet and Boost.ICL expose the arithmetic through the domain type. The library has no way to move a range or to grow it on both sides, so "the same slot one week later" and "the booking plus a 15-minute buffer on each side" are a deconstruct-and-rebuild per shape today.

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

• Shift(delta) — DateRange.Shift(int days), DateTimeRange.Shift(TimeSpan), DateTimeOffsetRange.Shift(TimeSpan), TimeRange.Shift(TimeSpan), Int32Range.Shift(int), Int64Range.Shift(long), DecimalRange.Shift(decimal); NodaTime LocalDateRange.Shift(Period), InstantRange.Shift(Duration), YearMonthRange.Shift(int months). Bounds and inclusiveness move together; Empty and Infinite return themselves; an unbounded side stays unbounded.
• Expand(amount) — grows both bounds outward by amount (a negative amount shrinks; a result that inverts is Empty, as CreateFinite already does).

Overflow at the extremes (int.MaxValue, DateOnly.MaxValue) throws rather than wrapping, like NextValueAfter returning null.

Translation: PostgreSQL has no range arithmetic, so the rebuilt constructor: int4range(lower(r) + @d, upper(r) + @d) for the discrete types (canonical [), so the flags are fixed), and for the continuous ones numrange(lower(r) + @d, upper(r) + @d, CASE … lower_inc/upper_inc … END) with the same flag handling CreateFinite uses (E08 requires constant flags today; this needs the CASE form or stays in-memory and says so).

Done when: Shift and Expand are in docs/ranges.md, held by claims, swept by the small-model oracle (Shift(d).Shift(-d) == this, Expand(a).Contains(this)), and either translated with a live test or listed under "runs in memory only" with the reason.

Found by the comparable-library survey (Intervals.NET), 2026-10-17.