feat: std.time — wall + monotonic clocks over clock_gettime (PLAN-HTTPZ S1)

now_secs (CLOCK_REALTIME, epoch seconds) and mono_ms (CLOCK_MONOTONIC,
process-local milliseconds for deadlines). Clock ids are darwin's; the
per-OS selection mechanism is PLAN-HTTPZ C3. No error channel: with
module-constant clock ids and a stack timespec, clock_gettime is total.
std.sx namespace tail carries the time alias; examples/1629 pins epoch
plausibility, monotone advance, and the alias carry.

library/modules/std.sx

@@ -97,3 +97,4 @@ cli     :: #import "modules/std/cli.sx";
 hash    :: #import "modules/std/hash.sx";
 log     :: #import "modules/std/log.sx";
 test    :: #import "modules/std/test.sx";
+time    :: #import "modules/std/time.sx";