config: centralize NYASH_STR_CP and add cp smoke

src/boxes/basic/string_box.rs

@@ -107,7 +107,7 @@ impl StringBox {
     /// otherwise use UTF-8 byte length (legacy/default).
     pub fn length(&self) -> Box<dyn NyashBox> {
         use crate::box_trait::IntegerBox;
-        let use_cp = std::env::var("NYASH_STR_CP").ok().as_deref() == Some("1");
+        let use_cp = crate::config::env::string_codepoint_mode();
         let n = if use_cp {
             self.value.chars().count() as i64
         } else {
@@ -144,10 +144,9 @@ impl StringBox {
 
     /// Get substring from start to end (exclusive)
     pub fn substring(&self, start: usize, end: usize) -> Box<dyn NyashBox> {
-        let chars: Vec<char> = self.value.chars().collect();
-        let actual_end = end.min(chars.len());
-        let actual_start = start.min(actual_end);
-        let substring: String = chars[actual_start..actual_end].iter().collect();
+        let mode = crate::boxes::string_ops::index_mode_from_env();
+        let substring =
+            crate::boxes::string_ops::substring(&self.value, start as i64, Some(end as i64), mode);
         Box::new(StringBox::new(substring))
     }
 }

src/boxes/string_ops.rs

@@ -7,7 +7,7 @@ pub enum StringIndexMode {
 }
 
 pub fn index_mode_from_env() -> StringIndexMode {
-    if std::env::var("NYASH_STR_CP").ok().as_deref() == Some("1") {
+    if crate::config::env::string_codepoint_mode() {
         StringIndexMode::CodePoint
     } else {
         StringIndexMode::Byte