Add shop components 436-450; Odometer exact at 15 fps

Fifteen shop components: PriceTag, Wallet, ShopItem, Cart, Receipt,
Checkout, CurrencyConvert, SaleBadge, Quantity, PricingTiers, Coupon,
DailyReward, BidTicker, TradeWindow and VendingMachine.

Receipt prints dot by dot with feed holes and a torn edge; Coupon cuts
real notches; SaleBadge is a sixteen-point starburst; VendingMachine
sells A1 to D4 by code and drops the snack into its tray. Sums are
checked against overflow, TradeWindow withdraws both confirms on any
change to an offer, and a disabled Checkout veils its inert content.

text.Odometer now counts digits as text, steps at 15 fps as S5 asks,
draws exact digits at rest and lands rolls that floats would stall.

text/text_test.go

@@ -337,7 +337,8 @@ func TestOdometerRollsToValue(t *testing.T) {
 	five := digits()
 	value = 6
 	tt.Frame()
-	clk.add(34 * time.Millisecond)
+	// One step at 15 fps, S5's fastest.
+	clk.add(67 * time.Millisecond)
 	tt.Frame()
 	mid := digits()
 	clk.add(time.Second)