Add insert command

internal/insert/parser.go

@@ -0,0 +1,46 @@
+package insert
+
+import (
+	"errors"
+	"fmt"
+	"unicode/utf8"
+)
+
+// ParseInputs validates the position token and insert text before preview/apply.
+func ParseInputs(positionToken, insertText string, stemLength int) error {
+	if positionToken == "" {
+		return errors.New("position token cannot be empty")
+	}
+	if insertText == "" {
+		return errors.New("insert text cannot be empty")
+	}
+	if !utf8.ValidString(insertText) {
+		return errors.New("insert text must be valid UTF-8")
+	}
+	for _, r := range insertText {
+		if r == '/' || r == '\\' {
+			return errors.New("insert text must not contain path separators")
+		}
+		if r < 0x20 {
+			return errors.New("insert text must not contain control characters")
+		}
+	}
+	if stemLength >= 0 {
+		if _, err := ResolvePosition(positionToken, stemLength); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// ResolvePositionWithValidation wraps ResolvePosition with explicit range checks.
+func ResolvePositionWithValidation(positionToken string, stemLength int) (Position, error) {
+	pos, err := ResolvePosition(positionToken, stemLength)
+	if err != nil {
+		return Position{}, err
+	}
+	if pos.Index < 0 || pos.Index > stemLength {
+		return Position{}, fmt.Errorf("position %s out of range for %d-character stem", positionToken, stemLength)
+	}
+	return pos, nil
+}