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/rhythm-grounding.el

@@ -0,0 +1,18 @@
+// rhythm-grounding.el — Mondays at 9am, grounded against EarthCalendar(NYC),
+// from a Wednesday timestamp returns the next Monday at 9am EDT.
+// Wednesday 2026-05-06 00:00 UTC (1778025600) → next Monday 9am EDT
+//   = 2026-05-11 09:00 EDT = 2026-05-11 13:00 UTC = 1778504400.
+
+fn run_test() -> Int {
+    let z: Zone = zone("America/New_York")
+    let cal: Calendar = earth_calendar(z)
+    let r: Rhythm = rhythm_weekly_at(1, 9, 0)
+    let after: Instant = unix_seconds(1778025600)
+    let next: Instant = rhythm_next_after(r, after, cal)
+    let next_secs: Int = instant_to_unix_seconds(next)
+    return next_secs
+}
+
+fn main() -> Void {
+    println(int_to_str(run_test()))
+}