fix: ensure UpperCaseWord preserves case in Verbatim mode

Previously, `UpperCaseWord` was forcibly lowercased in `WordsToFormattedCase`
when no specific casing option (like `CMScreaming` or `CMWhispering`) was provided.
This contradicted the behavior of `AcronymWord` and `ExactCaseWord`, which preserve
case in default/Verbatim mode.

This commit removes the fallback lowercasing for `UpperCaseWord`, ensuring that
`ToSnakeCase` and other formatters produce consistent results when `SmartAcronyms`
is disabled (which causes uppercase tokens to be classified as `UpperCaseWord`).

Added regression test `TestUpperCaseWord_Verbatim_Bug` in `types_test.go`.

Author: google-labs-jules[bot]

types.go

@@ -386,8 +386,6 @@ func WordsToFormattedCase(words []Word, opts ...any) (string, error) {
 				if err != nil {
 					return "", err
 				}
-			} else {
-				w = strings.ToLower(w)
 			}
 		case SeparatorWord:
 			w = word.String()

types_test.go

@@ -496,6 +496,7 @@ func TestOptionUTF8Modes(t *testing.T) {
 			},
 			options:   []Option{OptionCaseMode(CMAllTitle), OptionStrict()},
 			expectErr: true,
+			expected:  "",
 		},
 		{
 			name: "SingleCaseWord CMAllTitle Loose",
@@ -529,3 +530,28 @@ func TestOptionUTF8Modes(t *testing.T) {
 		})
 	}
 }
+
+func TestUpperCaseWord_Verbatim_Bug(t *testing.T) {
+	// "HELLO" is parsed as AcronymWord by default (SmartAcronyms=true).
+	// But if SmartAcronyms=false, it becomes UpperCaseWord.
+
+	input := "HELLO"
+
+	// Case 1: Default (SmartAcronyms=true)
+	words1, _ := Parse(input)       // [AcronymWord("HELLO")]
+	res1, _ := ToSnakeCase(words1) // ToSnakeCase defaults to Verbatim (but with delimiter "_")
+	// AcronymWord preserves case by default.
+	if res1 != "HELLO" {
+		t.Errorf("Default behavior changed? Got %q, want %q", res1, "HELLO")
+	}
+
+	// Case 2: SmartAcronyms=false
+	words2, _ := Parse(input, WithSmartAcronyms(false)) // [UpperCaseWord("HELLO")]
+	// Expectation: Verbatim mode should preserve case -> "HELLO"
+	res2, _ := ToSnakeCase(words2)
+
+	expected := "HELLO"
+	if res2 != expected {
+		t.Errorf("UpperCaseWord (SmartAcronyms=false) did not preserve case. Got %q, want %q", res2, expected)
+	}
+}