test: improve mutation coverage across 4 modules

Mutation testing (56 mutations, 10 modules) identified 7 surviving
mutations. Added 5 targeted tests to kill all actionable survivors.
Remaining 2 are logically equivalent condition reorderings in
filter.go. Overall mutation score: 96.4%.

New tests:
- TestHasMalformedYAML: YAML unmarshal error path
- TestInvalidateInvalidCheatpath: cheatpath.Validate() delegation
- TestIndentTrimsWhitespace: TrimSpace behavior
- TestColorizeDefaultSyntax: default "bash" lexer
- TestColorizeExplicitSyntax: lexer differentiation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/display/indent_test.go

@@ -10,3 +10,13 @@ func TestIndent(t *testing.T) {
 		t.Errorf("failed to indent: want: %s, got: %s", want, got)
 	}
 }
+
+// TestIndentTrimsWhitespace asserts that Indent trims leading and trailing
+// whitespace before indenting
+func TestIndentTrimsWhitespace(t *testing.T) {
+	got := Indent("  foo\nbar\nbaz  \n")
+	want := "\tfoo\n\tbar\n\tbaz\n"
+	if got != want {
+		t.Errorf("failed to trim and indent: want: %q, got: %q", want, got)
+	}
+}