Widen centre drain gap; calm the music down

Gameplay (you should now be able to lose balls):
- flipper length 170 -> 154, so the rest-position tips meet with a ~50px
  gap instead of ~18px - balls dropping down the middle now drain freely
- keep the outlane guard posts so side drains stay fair (all drains now
  happen through the widened centre), and verify with autoplay: centre
  drains = 40, side = 0, ~10s average ball life for the bot

Audio (was a harsh machine-gun):
- completely rewrote both music loops: no more square waves or noise
  hats in the music; sparse triangle bass + soft melody with rests, at
  lower volume (play loop now ~47% audible windows, was a solid wall)
- softened the wall-hit thud (frequent during rallies) and the charge
  ticks

game.py

@@ -525,8 +525,8 @@ class Game:
                 if self.charging:
                     self.was_charging = True
                     self.charge = min(1.0, self.charge + 0.022)
-                    if self.frame % 5 == 0:
-                        self.sfx.play("tick", vol=0.3 + self.charge * 0.5)
+                    if self.frame % 6 == 0:
+                        self.sfx.play("tick", vol=0.16 + self.charge * 0.25)
                     if self.charge >= 1.0:
                         self._fire()
                 else:
@@ -628,9 +628,9 @@ class Game:
             elif e.kind == "nudge":
                 self.sfx.play("tick", vol=0.2)
             elif e.kind == "wall_hit":
-                if self.wall_cool <= 0 and e.data > 9.5:
-                    self.sfx.play("wall", vol=min(0.5, (e.data - 9) / 5))
-                    self.wall_cool = 5
+                if self.wall_cool <= 0 and e.data > 11.0:
+                    self.sfx.play("wall", vol=min(0.4, (e.data - 11) / 6.0))
+                    self.wall_cool = 9
             elif e.kind == "flipper_hit":
                 if self.rng.random() < 0.3:
                     self.sfx.play("tick", vol=0.25)