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.

examples/expected/1629-std-time.exit

@@ -0,0 +1 @@
+0

examples/expected/1629-std-time.stderr

@@ -0,0 +1 @@
+

examples/expected/1629-std-time.stdout

@@ -0,0 +1 @@
+std.time ok