Play rival encounter music when rival leaves Oak's Lab after battle

The parcel scene in Oak's Lab plays Music_MeetRival on both the rival's
arrival and departure (lines 144-146 in oaks_lab.lua), but the post-battle
onStep exit sequence only played the fanfare when the rival approached
(fixed in #596). It was missing when the rival walks out after the battle.

Add stop_music + play_music Music_MeetRival before the rival's exit
walk-out in both oaks_lab.lua and oaks_lab_yellow.lua, matching the
parcel scene's double-fanfare pattern from the original ROM.

Fixes #683

data/scripts/oaks_lab.lua

@@ -325,9 +325,14 @@ return {
       table.insert(rows, { "jump_if_false", base + 6 })
       table.insert(rows, { "show_text", "_OaksLabRivalIPickedTheWrongPokemonText" })
       table.insert(rows, { "show_text", "_OaksLabRivalSmellYouLaterText" })
+      -- OaksLabRivalStartsExitScript: parting shot, rival exit fanfare, then
+      -- walk out past the player.  The fanfare was dropped here (#683) -- the
+      -- parcel scene above already plays Music_MeetRival on both arrival and
+      -- departure (lines 144-146), and this exit should match (#596).
+      table.insert(rows, { "stop_music" })
+      table.insert(rows, { "play_music", "Music_MeetRival" })
       table.insert(rows, { "move_npc_to", 1, 4, 11 })
       table.insert(rows, { "hide_object", "OAKS_LAB", "OAKSLAB_RIVAL" })
-      -- restore the lab theme once he's walked out, same as the Yellow port
       table.insert(rows, { "play_music", "Music_OaksLab" })
       ow.runner:run(rows, { npc = rival })
       return true

data/scripts/oaks_lab_yellow.lua

@@ -287,10 +287,12 @@ return {
       table.insert(rows, { "label", "lost_lab" })
       table.insert(rows, { "set_field", "rivalStarter", 3 })
       table.insert(rows, { "label", "exit" })
-      -- OaksLabRivalStartsExitScript: parting shot, walk out past the
-      -- player, restore the lab theme
+      -- OaksLabRivalStartsExitScript: parting shot, rival exit fanfare, then
+      -- walk out past the player (#683).
       table.insert(rows, { "wait", 20 })
       table.insert(rows, { "show_text", "_OaksLabRivalSmellYouLaterText" })
+      table.insert(rows, { "stop_music" })
+      table.insert(rows, { "play_music", "Music_MeetRival" })
       table.insert(rows, { "move_npc_to", RIVAL, 4, 11 })
       table.insert(rows, { "hide_object", "OAKS_LAB", "OAKSLAB_RIVAL" })
       table.insert(rows, { "play_music", "Music_OaksLab" })