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