Refine Korean briefing and expand first battle

tools/validate_data.ps1

@@ -1000,6 +1000,10 @@ function Check-Terrain-Definitions() {
         if ($avoid -lt 0 -or $avoid -gt 100) {
             Fail "Terrain $terrainKey avoid must be between 0 and 100."
         }
+        $heal = [int](Get-Prop $terrainDef "heal" 0)
+        if ($heal -lt 0 -or $heal -gt 99) {
+            Fail "Terrain $terrainKey heal must be between 0 and 99."
+        }
     }
 }