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-literals.el

@@ -0,0 +1,15 @@
+// time-literals.el — postfix-literal duration recognition.
+// `30.seconds` lowers to a DurationLit AST node carrying the count and the
+// unit; codegen emits a literal int64 nanosecond constant. duration_to_seconds
+// reverses the lowering and yields the original count.
+
+// `test` is reserved by El's lexer as a property-test keyword, so the
+// example body lives in `run_test()`.
+fn run_test() -> Int {
+    let d: Duration = 30.seconds
+    return duration_to_seconds(d)
+}
+
+fn main() -> Void {
+    println(int_to_str(run_test()))
+}