add Instant + Duration as first-class types in El

Postfix literal syntax (30.seconds, 1.hour). Type-checked arithmetic
(Instant + Duration -> Instant; Instant + Instant fails). TTL helpers
backed by typed Duration. sleep(Duration) replaces ambiguous sleep(Int).

Self-host fixed point holds at 5797 lines.
Snapshot tagged at dist/platform/elc.20260502-1256-self-host.

Phase 2 (every/after/at scheduling primitives) lands separately.
Backlog: bl-937e9c30

tests/time/examples/time-comparison.el

@@ -0,0 +1,16 @@
+// time-comparison.el — Duration < Duration is type-checked at codegen time.
+// Both operands are DurationLits (postfix literals). Codegen routes through
+// el_duration_lt; the runtime returns 1 when 5.minutes < 1.hour holds.
+
+// `test` is reserved; using `run_test` instead.
+fn run_test() -> Bool {
+    return 5.minutes < 1.hour
+}
+
+fn main() -> Void {
+    if run_test() {
+        println("1")
+    } else {
+        println("0")
+    }
+}