fix: add 15 stretching exercises to exercise database

Exercises used by the Day 6 stretching template were only in
exercisedb-map.ts but missing from exercises.ts, causing the
template detail to show raw IDs instead of proper names.

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "homepage",
-	"version": "1.25.2",
+	"version": "1.25.3",
 	"private": true,
 	"type": "module",
 	"scripts": {