Song select: a howto board for the tutorial folder

The tutorial folder (GENRE:TUTORIAL) uses the recent board's colours with its
own characters (box_chara frame 10).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Scripts/song_select/file_navigator/box_folder.lua

@@ -26,6 +26,15 @@ local SORT_FOLDER_NAMES  = { ["曲検索"] = true, ["Search Song"] = true, ["搜
                              ["むずかしさからえらぶ"] = true, ["Diff Sort"] = true }
 local FOLDER_FRAME_EVENT, FOLDER_FRAME_SORT, FOLDER_FRAME_DANI = 11, 12, 10
 
+-- The tutorial folder (GENRE:TUTORIAL, genre index 0) is a howto board: the recent
+-- board's colours with its own characters (box_chara frame 10).
+local GENRE_TUTORIAL = 0
+local FOLDER_FRAME_HOWTO, CHARA_FRAME_HOWTO = 9, 10
+
+local function is_tutorial(box)
+    return box.genre_index ~= nil and box:genre_index() == GENRE_TUTORIAL
+end
+
 local SORT_COLLECTIONS  = { SEARCH = true, DIFFICULTY = true }
 local EVENT_COLLECTIONS = { NEW = true, RECOMMENDED = true }
 
@@ -39,6 +48,7 @@ local function folder_frame(box)
     end
     if box.kind and box:kind() == "dan" then return FOLDER_FRAME_DANI, false end
     if n and SORT_FOLDER_NAMES[n]  then return FOLDER_FRAME_SORT, false end
+    if is_tutorial(box) then return FOLDER_FRAME_HOWTO, false end
     return box:genre_frame(), false
 end
 
@@ -115,7 +125,7 @@ function SongSelect:draw_folder_board(box, pb, ch_alpha, ch_off, ta, glow_a)
     -- characters: texture has L at x=510 and R at x=1410
     if ch_alpha > 0 then
         local slide = 450.0 - ch_off
-        local gframe = box:genre_frame()
+        local gframe = is_tutorial(box) and CHARA_FRAME_HOWTO or box:genre_frame()
         tex.draw_texture(self._id_box_chara, { frame = gframe, x = bx - 960 + slide, y = by - 240, x2 = 960 - 1920, fade = fade * ch_alpha,
                                                src = { x = 0, y = 0, width = 960, height = 480 } })
         tex.draw_texture(self._id_box_chara, { frame = gframe, x = bx - slide, y = by - 240, x2 = 960 - 1920, fade = fade * ch_alpha,