[Repo Assist] feat: add calendar module bindings and tests

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What

Adds Fable.Beam.Calendar — F# bindings for Erlang's calendar module (stdlib), covering date/time introspection and Gregorian calendar arithmetic.

Changes

src/otp/Calendar.fs (new)

Type aliases (for readability):

• type Date = int * int * int — (Year, Month, Day)
• type Time = int * int * int — (Hour, Min, Sec)
• type DateTime = Date * Time — ((Year,Month,Day),(Hour,Min,Sec))

[<ImportAll>] bindings (multi-arg variants where available):

Function	Description
calendar.date()	Current local date
calendar.time()	Current local time
calendar.local_time()	Current local datetime
calendar.universal_time()	Current UTC datetime
calendar.day_of_the_week(year, month, day)	Day number (1=Mon, 7=Sun)
calendar.is_leap_year(year)	Leap year check
calendar.last_day_of_the_month(year, month)	Last day of month
calendar.date_to_gregorian_days(year, month, day)	Date → Gregorian day count
calendar.gregorian_days_to_date(days)	Gregorian day count → Date
calendar.gregorian_seconds_to_datetime(seconds)	Gregorian seconds → DateTime

[<Emit>] bindings (functions that take single tuple args on the Erlang side):

Function	Description
datetimeToGregorianSeconds(datetime)	DateTime → Gregorian second count (int64)
timeToSeconds(time)	Time → seconds since midnight
secondsToTime(seconds)	Seconds since midnight → Time
localTimeToUniversalTime(datetime)	Local → UTC
universalTimeToLocalTime(datetime)	UTC → Local

test/TestCalendar.fs (new) — 24 tests

• is_leap_year: 2000 ✓, 2024 ✓, 1900 ✗ (century rule), 2023 ✗
• last_day_of_the_month: Jan=31, Feb leap=29, Feb non-leap=28, Apr=30
• day_of_the_week: Mon/Fri/Sun for known 2024 dates
• date_to_gregorian_days / gregorian_days_to_date: roundtrip + known value (2000-01-01 = day 730485)
• timeToSeconds / secondsToTime: midnight, noon, 01:30:30 roundtrips
• datetimeToGregorianSeconds / gregorian_seconds_to_datetime: roundtrip + known value (2000-01-01 00:00:00 = 63113904000)
• date, time, local_time, universal_time: sanity-range checks on live values

Other

• src/Fable.Beam.fsproj: added Calendar.fs after Re.fs
• test/Fable.Beam.Test.fsproj: added TestCalendar.fs after TestRe.fs
• README.md: added Fable.Beam.Calendar and Fable.Beam.Re to module table

Notes

• datetimeToGregorianSeconds returns int64 because Gregorian seconds for modern dates exceed 231 (e.g. 2000-01-01 = 63,113,904,000).
• localTimeToUniversalTime / universalTimeToLocalTime results are timezone-dependent; no test asserts specific UTC offset.
• F# compilation verified locally with dotnet build test/. Full BEAM validation by CI.

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