fix: truncate() now handles multi-byte characters correctly

The Truncate function was using byte slicing (str[:n]) which corrupts
multi-byte UTF-8 characters like emojis. Changed to rune slicing to
preserve character integrity when truncating strings.

Before: truncate("hello😀world", 7) → "hello\xf0…" (corrupted)
After:  truncate("hello😀world", 7) → "hello😀…" (correct)

Author: amterp

core/common/utils_rad.go

@@ -1,14 +1,12 @@
 package com
 
 func Truncate(str string, maxLen int64) string {
+	runes := []rune(str)
 	if TerminalIsUtf8 {
-		str = str[:maxLen-1]
-		str += "…"
+		return string(runes[:maxLen-1]) + "…"
 	} else {
-		str = str[:maxLen-3]
-		str += "..."
+		return string(runes[:maxLen-3]) + "..."
 	}
-	return str
 }
 
 func Reverse(str string) string {

core/funcs.go

@@ -594,8 +594,14 @@ func init() {
 			Execute: func(f FuncInvocation) RadValue {
 				str := f.GetStr("_str")
 				maxLen := f.GetInt("_len")
-				if maxLen < 0 {
-					return f.ReturnErrf(rl.ErrNumInvalidRange, "Requires a non-negative int, got %d", maxLen)
+
+				// Minimum length depends on ellipsis: "…" (1 char) vs "..." (3 chars)
+				minLen := int64(1)
+				if !com.TerminalIsUtf8 {
+					minLen = 3
+				}
+				if maxLen < minLen {
+					return f.ReturnErrf(rl.ErrNumInvalidRange, "Requires at least %d, got %d", minLen, maxLen)
 				}
 
 				strLen := str.Len()

core/testing/func_truncate_test.go

@@ -0,0 +1,69 @@
+package testing
+
+import "testing"
+
+func Test_Truncate_MultiByte(t *testing.T) {
+	script := `
+s = "hello😀world"
+print(truncate(s, 7))
+`
+	setupAndRunCode(t, script, "--color=never")
+	assertOnlyOutput(t, stdOutBuffer, "hello😀…\n")
+	assertNoErrors(t)
+}
+
+func Test_Truncate_MultiByte_ExactBoundary(t *testing.T) {
+	script := `
+s = "a😀b"
+print(truncate(s, 2))
+`
+	setupAndRunCode(t, script, "--color=never")
+	assertOnlyOutput(t, stdOutBuffer, "a…\n")
+	assertNoErrors(t)
+}
+
+func Test_Truncate_AllEmoji(t *testing.T) {
+	script := `
+s = "😀😀😀😀😀"
+print(truncate(s, 3))
+`
+	setupAndRunCode(t, script, "--color=never")
+	assertOnlyOutput(t, stdOutBuffer, "😀😀…\n")
+	assertNoErrors(t)
+}
+
+func Test_Truncate_MinLength(t *testing.T) {
+	// Minimum length is 1 (for UTF-8 ellipsis "…")
+	script := `
+print(truncate("hello", 1))
+`
+	setupAndRunCode(t, script, "--color=never")
+	assertOnlyOutput(t, stdOutBuffer, "…\n")
+	assertNoErrors(t)
+}
+
+func Test_Truncate_ErrorsForZero(t *testing.T) {
+	script := `
+print(truncate("hello", 0))
+`
+	setupAndRunCode(t, script, "--color=never")
+	expected := `Error at L2:7
+
+  print(truncate("hello", 0))
+        ^^^^^^^^^^^^^^^^^^^^ Requires at least 1, got 0 (RAD20017)
+`
+	assertError(t, 1, expected)
+}
+
+func Test_Truncate_ErrorsForNegative(t *testing.T) {
+	script := `
+print(truncate("hello", -5))
+`
+	setupAndRunCode(t, script, "--color=never")
+	expected := `Error at L2:7
+
+  print(truncate("hello", -5))
+        ^^^^^^^^^^^^^^^^^^^^^ Requires at least 1, got -5 (RAD20017)
+`
+	assertError(t, 1, expected)
+}