exthost.platform: the frozen window opens once the freeze has landed

The test took its first reading of the frozen probe's cpu.stat the moment
cgroup.events said frozen 1, and wanted 0 us over the next 1.5 s. The
kernel reports a task frozen before its last way into schedule() is
charged to the group, so a reading at the event can come before that
charge lands. On image 20260928170058's campaign the frozen probe was
charged 16 us and the test failed. The first reading now comes 0.2 s
after the event; the window and its judgment (0 us) are unchanged.

Proof: pyflakes clean. On the bench reference, a spinner under cpu.max
5% in a cgroup of its own, frozen 520 times with the first reading taken
both ways: read at the event (polled every 50 ms, as the test does), the
next 0.5 s was charged 4 times (13 to 20 us); read after the 0.2 s
settle, never.

Acceptance: the change is exthost.platform itself, inside its own
function, so no other test's fingerprint moves.

forgetest/forgetest/suite/exthost.py

@@ -339,6 +339,9 @@ def platform(ctx):
         _write(PROBE_GROUP + "/cgroup.freeze", "1")
         ok = ctx.wait_for(lambda: _flat(PROBE_GROUP + "/cgroup.events").get("frozen") == 1, 3, poll=0.05)
         ctx.check(ok is not None, "the group did not freeze: %s", _flat(PROBE_GROUP + "/cgroup.events"))
+        # The kernel reports a task frozen before its last way into
+        # schedule() is charged, so the window opens once that has landed.
+        ctx.sleep(0.2)
         f0 = _flat(PROBE_GROUP + "/cpu.stat")["usage_usec"]
         ctx.sleep(1.5)
         f1 = _flat(PROBE_GROUP + "/cpu.stat")["usage_usec"]