add Calendar + CalendarTime + Rhythm + LocalDate/Time as first-class

Phase 1.5 of time-system. Calendar is pluggable: EarthCalendar
(IANA zones, DST, Gregorian) is the default; MarsCalendar,
CycleCalendar(period), NoCycleCalendar handle non-Earth cases.

Rhythm abstracts recurrence from clock units - rhythm_cycle_phase(0.5)
means "midpoint of cycle" whether the cycle is 24 hours on Earth or
30 hours on a station or 300 years on a long-cycle world.

Phase 1 (Instant + Duration) unchanged. EarthCalendar(zone_local())
is the user-facing default; nobody who doesn't care about non-Earth
calendars sees the abstraction.

Self-host fixed point holds at 6339 lines.
Snapshot tagged at dist/platform/elc.20260502-1321-self-host.

Phase 2 (scheduling primitives every/after/at) lands next, now with
Calendar-aware grounding instead of Earth-time hardcoded.

Backlog: bl-297f66d8 (supersedes bl-b29b3e60)

tests/calendar/examples/cycle-30hr.el

@@ -0,0 +1,20 @@
+// cycle-30hr.el — CycleCalendar with a 30-hour period.
+// Two CalendarTimes 15 hours apart should have cycle_phase differ by 0.5.
+// We compare phases via float subtraction with format_float for determinism.
+
+fn run_test() -> String {
+    let period: Duration = 30.hours
+    let cal: Calendar = cycle_calendar(period)
+    let base: Instant = unix_seconds(0)
+    let later: Instant = base + 15.hours
+    let ct1: CalendarTime = in_calendar(base, cal)
+    let ct2: CalendarTime = in_calendar(later, cal)
+    let p1: Float = cal_cycle_phase(ct1)
+    let p2: Float = cal_cycle_phase(ct2)
+    let diff: Float = p2 - p1
+    return format_float(diff, 1)
+}
+
+fn main() -> Void {
+    println(run_test())
+}