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/ttl-cache.el

@@ -0,0 +1,14 @@
+// ttl-cache.el — TTL helpers backed by typed Duration. Storing under a key
+// stamps the set time as an Instant; ttl_cache_get computes the age and
+// returns the value only if the age is within the supplied Duration.
+
+// `test` is reserved; using `run_test` instead.
+fn run_test() -> String {
+    ttl_cache_set("key1", "hello")
+    let v: String = ttl_cache_get("key1", 1.hour)
+    return v
+}
+
+fn main() -> Void {
+    println(run_test())
+}