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/1629-std-time.sx

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+// std.time (PLAN-HTTPZ S1): the wall clock reads a plausible epoch and
+// the monotonic clock advances and never runs backwards. The `time`
+// alias rides the std.sx namespace tail like its siblings.
+#import "modules/std.sx";
+
+main :: () -> i32 {
+    now := time.now_secs();
+    if now < 1767225600 {            // before 2026-01-01: implausible
+        print("wall clock implausibly old: {}\n", now);
+        return 1;
+    }
+    if now > 4102444800 {            // after 2100-01-01: implausible
+        print("wall clock implausibly far: {}\n", now);
+        return 1;
+    }
+
+    a := time.mono_ms();
+    b := a;
+    spins := 0;
+    while b == a and spins < 100000000 {   // a real ms tick arrives long before the bound
+        b = time.mono_ms();
+        spins += 1;
+    }
+    if b < a {
+        print("monotonic went backwards: {} -> {}\n", a, b);
+        return 1;
+    }
+    if b == a {
+        print("monotonic never advanced\n");
+        return 1;
+    }
+
+    print("std.time ok\n");
+    return 0;
+}

examples/expected/1629-std-time.exit

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

examples/expected/1629-std-time.stderr

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+

examples/expected/1629-std-time.stdout

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+std.time ok