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/no-cycle.el

@@ -0,0 +1,18 @@
+// no-cycle.el — NoCycleCalendar's cycle_phase returns NaN sentinel.
+// Detection via float_to_str — NaN renders as "nan" under %g.
+
+fn run_test() -> Int {
+    let cal: Calendar = no_cycle_calendar()
+    let i: Instant = unix_seconds(1000000000)
+    let ct: CalendarTime = in_calendar(i, cal)
+    let p: Float = cal_cycle_phase(ct)
+    let s: String = float_to_str(p)
+    if str_eq(s, "nan") {
+        return 1
+    }
+    return 0
+}
+
+fn main() -> Void {
+    println(int_to_str(run_test()))
+}