feat: 更新插入命令以支持尾部偏移标记和相关文档

internal/insert/positions.go

@@ -3,6 +3,7 @@ package insert
 import (
 	"errors"
 	"fmt"
+	"strings"
 	"unicode/utf8"
 )
 
@@ -11,8 +12,12 @@ type Position struct {
 	Index int // zero-based index where insertion should occur
 }
 
-// ResolvePosition interprets a position token (`^`, `$`, positive, negative) against the stem length.
+// ResolvePosition interprets a position token (`^`, `$`, forward indexes, suffix offsets like `N$`, or legacy negative values) against the stem length.
 func ResolvePosition(token string, stemLength int) (Position, error) {
+	if token == "" {
+		return Position{}, errors.New("position token cannot be empty")
+	}
+
 	switch token {
 	case "^":
 		return Position{Index: 0}, nil
@@ -20,8 +25,40 @@ func ResolvePosition(token string, stemLength int) (Position, error) {
 		return Position{Index: stemLength}, nil
 	}
 
-	if token == "" {
-		return Position{}, errors.New("position token cannot be empty")
+	if strings.HasPrefix(token, "^") {
+		trimmed := token[1:]
+		if trimmed == "" {
+			return Position{Index: 0}, nil
+		}
+		value, err := parseInt(trimmed)
+		if err != nil {
+			return Position{}, fmt.Errorf("invalid position token %q: %w", token, err)
+		}
+		if value < 0 {
+			return Position{}, fmt.Errorf("invalid position token %q: cannot be negative", token)
+		}
+		if value > stemLength {
+			return Position{}, fmt.Errorf("position %d out of range for %d-character stem", value, stemLength)
+		}
+		return Position{Index: value}, nil
+	}
+
+	if strings.HasSuffix(token, "$") {
+		trimmed := token[:len(token)-1]
+		if trimmed == "" {
+			return Position{Index: stemLength}, nil
+		}
+		value, err := parseInt(trimmed)
+		if err != nil {
+			return Position{}, fmt.Errorf("invalid position token %q: %w", token, err)
+		}
+		if value < 0 {
+			return Position{}, fmt.Errorf("invalid position token %q: cannot be negative", token)
+		}
+		if value > stemLength {
+			return Position{}, fmt.Errorf("position %d out of range for %d-character stem", value, stemLength)
+		}
+		return Position{Index: stemLength - value}, nil
 	}
 
 	// Try parsing as integer (positive or negative).