fix(gen2): sell TMs at Poké Marts instead of opening the teach party

Selecting a TM in the mart's SELL pack (and the item PC's DEPOSIT pack) opened the teach-party screen: PackMenu:useSelected taught any item carrying `teaches`, even when the pack was built as a chooser (DepositSellPack, world = {}). On the cart that chooser's jumptable is four ScrollingMenus and never reaches tmhm.asm, so the row must hand back to the caller instead.

Gate the teach and field-NOUSE branches behind world.useFieldItem so only the real field PACK teaches; choosers now hand TMs to onChoose. Selling then prices a TM at half its ItemAttributes price -- exactly half of what the Goldenrod/Celadon TM shelves charge, and half of the hidden price for the rest, the way SelectQuantityToSell -> GetItemPrice -> Sell_HalvePrice does on the cart.

Closes #1243

src/ui/gen2/MartMenu.lua

@@ -780,6 +780,11 @@ function MartMenu:offerToSell(itemId, count)
   end
   -- ScrollingMenu's .a_button copies the row's own quantity into
   -- wItemQuantity, so the selector's ceiling is how many you hold.
+  -- The unit price is the item's own ItemAttributes price: GetItemPrice is
+  -- what SelectQuantityToSell halves AND what the buy list's
+  -- GetMartItemPrice charges, so a TM sells for exactly half of what the
+  -- Goldenrod/Celadon TM shelves ask for it, and half of its hidden price
+  -- (usually ¥1,500) anywhere else (issue #1243).
   self.qtyItem = { id = itemId, name = def.name or itemId,
     price = def.price or 0 }
   self.qty = 1

src/ui/gen2/PackMenu.lua

@@ -354,20 +354,23 @@ function PackMenu:useSelected()
     end
     return
   end
-  -- engine/items/tmhm.asm:73
-  local def = self.items and self.items[row.id]
-  if def and def.teaches then
-    self:openTeachParty(row)
-    return
-  end
-  -- UseItem's jumptable runs off ITEMATTR's field-menu nibble, and the first
-  -- four entries are all .Oak -- an X ATTACK or a POKé DOLL used from the
-  -- field PACK prints OakThisIsntTheTimeText and goes nowhere.  Only the
-  -- FIELD pack owns that refusal: the battle pack returned above, and the
-  -- catch tutorial's DUDE pack carries a stub world with no useFieldItem at
-  -- all -- its POKE BALL is field-NOUSE and must still reach the throw.
+  -- UseItem's FIELD-pack tail (engine/items/tmhm.asm:73): a TM/HM row opens
+  -- the party to teach, and a field-NOUSE item -- an X ATTACK or a POKé DOLL
+  -- used from the field PACK -- prints OakThisIsntTheTimeText and goes
+  -- nowhere (UseItem's jumptable's first four entries are all .Oak).  Both
+  -- checks live behind `world.useFieldItem` on purpose: DepositSellPack (the
+  -- mart's SELL, the item PC's DEPOSIT) is a chooser whose jumptable is four
+  -- ScrollingMenus and never reaches tmhm.asm, so a TM picked there hands
+  -- its row to onChoose like every other item instead of opening the teach
+  -- party.  The battle pack returned above, and the catch tutorial's DUDE
+  -- pack carries a stub world with no useFieldItem at all -- its POKE BALL
+  -- is field-NOUSE and must still reach the throw.
   if world and world.useFieldItem then
     local def = self.items and self.items[row.id]
+    if def and def.teaches then
+      self:openTeachParty(row)
+      return
+    end
     if def and def.fieldMenu == "ITEMMENU_NOUSE" then
       self.message = OAK_THIS_ISNT_THE_TIME
       return