- fix: Correct sun.TrueBo calculation, now using basic.HSunTrueBo for correct solar latitude

- fix: Rename GetSunDownTime to GetSunSetTime in basic/sun.go and update related calls/outputs
- fix: Update test cases to use new method and error constant names

- improve: Rename all DownTime/Down functions to SetTime/Set for consistency
- improve: Standardize ERR_XXX_NEVER_DOWN error constants to ERR_XXX_NEVER_SET, with ERR_XXX_NEVER_DOWN kept as compatibility alias
- improve: More standard naming for interfaces and errors to improve maintainability and readability

neptune/neptune_test.go

@@ -22,5 +22,5 @@ func TestNeptune(t *testing.T) {
 	if date.Unix() != 1642644398 {
 		t.Fatal(date.Unix())
 	}
-	fmt.Println(DownTime(date, 115, 40, 0, false))
+	fmt.Println(SetTime(date, 115, 40, 0, false))
 }