lib: split time_test into duration/instant/arithm tests per ref/hare/time

Mirror ref/hare/time's impl layout in the test siblings: constants ->
duration_test.ww (ref/hare/time/duration.ha:9-18 owns the constants);
now/sleep rows -> instant_test.ww (instant is the subject type,
ref/hare/time/instant.ha; now/sleep bodies live in
ref/hare/time/+linux/functions.ha, a platform split ww does not have);
add/diff/compare rows -> arithm_test.ww (ref/hare/time/arithm.ha).

Pure move: every @test block is byte-identical; only the section
banner lines are deleted (the file names now carry them). The stale
file header naming the retired `ww run lib/time/timetest.ww` entry
point is dropped, not relocated (rule 8: it described a WHAT that no
longer exists).

Consumers: Makefile LIBRARY_TESTS entry replaced in place;
test/byteid roster 44->46 (+2).

lib/time/instant_test.ww

@@ -0,0 +1,33 @@
+package time_test;
+
+import time;
+
+@test fn nowmonotonic() void = {
+	let a = time.now(time.clock.monotonic);
+	let b = time.now(time.clock.monotonic);
+	// Two back-to-back calls — b can equal a (same ns tick) but
+	// must never precede it.
+	assert(!(time.compare(a, b) > 0i8));
+};
+
+// Sanity check that the syscall plumbing actually lands real bytes
+// in the instant. 2020-01-01 UTC = 1577836800 unix seconds; any
+// past-2020 wall clock satisfies this. (CI / test machines running
+// pre-2020 would fail here; acceptable trade for catching a
+// zero-init / wrong-slot regression.)
+
+@test fn nowrealtime() void = {
+	let t = time.now(time.clock.realtime);
+	assert(!(t.sec < 1577836800i64));
+	assert(!(t.nsec < 0i64));
+	assert(!(t.nsec >= 1000000000i64));
+};
+
+@test fn sleepmonotonic() void = {
+	let before: time.instant = time.now(time.clock.monotonic);
+	time.sleep((5i64 * (time.millisecond: i64)): time.duration,
+	    time.clock.monotonic);
+	let after: time.instant = time.now(time.clock.monotonic);
+	let elapsed: i64 = time.diff(before, after): i64;
+	assert(!(elapsed < 5i64 * (time.millisecond: i64)));
+};