calendar, mavweb: read the notification clock as his wall clock (V-482)

A phone posts an RFC 3339 instant ending in Z, and the clock inside the
text is a wall clock nobody means in UTC. The wall clock used to be
resolved against Posted's own zone, so on this UTC+4 box a 14:30 standup
was stored at 18:30. The size of the error is the deploy's offset, which
is why the tests never saw it: they ran on a UTC box.

EventFromNotificationIn takes the zone explicitly and EventFromNotification
passes time.Local. The day comes from Posted's local day too, since a
notification posted at 23:30Z saying "завтра" is already tomorrow where he
is standing. Posted itself stays an instant, so the past-grace check still
compares instants.

The two handler fixtures said a bare "10:00" against a 09:40Z post, which
is stale once the clock is read locally. They say "завтра" now, so they
mean a future meeting in every zone. internal/calendar and cmd/mavweb pass
under UTC, Europe/Samara, America/Los_Angeles, Pacific/Kiritimati and
Asia/Kathmandu.

cmd/mavweb/ambient_test.go

@@ -139,7 +139,10 @@ func TestHandleAmbientIgnoresNonMeetings(t *testing.T) {
 }
 
 func TestHandleAmbientAuth(t *testing.T) {
-	body := `{"title":"Планёрка 10:00","posted_at":"2026-08-03T09:40:00Z"}`
+	// "завтра" so the meeting is in the future in every zone: the clock in the
+	// text is a local wall clock and posted_at is a Z instant, so a bare "10:00"
+	// is already stale on a box east of UTC and stores nothing.
+	body := `{"title":"Планёрка завтра 10:00","posted_at":"2026-08-03T09:40:00Z"}`
 
 	newReq := func(hdr, val string) *http.Request {
 		r := httptest.NewRequest(http.MethodPost, "/api/ambient", strings.NewReader(body))
@@ -245,3 +248,31 @@ func TestHandleAmbientBadInput(t *testing.T) {
 		}
 	})
 }
+
+// The Z-instant defect end to end: a phone posts an RFC 3339 instant in UTC and
+// the clock inside the text is his wall clock. On a UTC+4 box a 14:30 standup
+// used to be stored at 18:30, and the size of the error was the deploy's offset.
+func TestHandleAmbientStoresTheWallClockHeRead(t *testing.T) {
+	// No zone juggling: the assertion is that the stored wall clock is the one
+	// he read, whatever zone the box is in. That is false under the old code
+	// on every box except a UTC one.
+	core := &ambientCore{}
+	rr, resp := postAmbient(t, core, ambientTestToken, calendar.Notification{
+		Package: "com.slack",
+		Title:   "Standup",
+		Text:    "созвон завтра в 14:30",
+		Posted:  time.Date(2026, 8, 2, 9, 0, 0, 0, time.UTC),
+	})
+	if rr.Code != http.StatusCreated {
+		t.Fatalf("status = %d, want 201: %s", rr.Code, rr.Body)
+	}
+	if !strings.HasSuffix(resp.Key, "_Standup") {
+		t.Errorf("key = %q, want a key naming the meeting", resp.Key)
+	}
+	if len(core.writeLog) != 1 {
+		t.Fatalf("expected 1 fact write, got %d", len(core.writeLog))
+	}
+	if got := core.writeLog[0].Value; got != "Standup @ 14:30-15:00" {
+		t.Errorf("value = %q, want %q", got, "Standup @ 14:30-15:00")
+	}
+}