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

physics.py

@@ -17,7 +17,7 @@ MAX_SPEED = 13.0          # speed cap (prevents tunneling; ~300px of rise)
 RESTITUTION = 0.42
 BALL_R = 7.0
 
-FLIP_LEN = 170.0
+FLIP_LEN = 154.0
 FLIP_REST = math.radians(6.0)      # tips slightly lowered toward center drain
 FLIP_UP = -math.radians(52.0)      # fired position
 FLIP_PIVOT_L = (62.0, 338.0)
@@ -36,10 +36,11 @@ BUMPERS = [
 SLING_KICK_L = (0.60, -0.80)
 SLING_KICK_R = (-0.60, -0.80)
 SLINGS = [
-    # upper slingshots by the lower bumpers - kick toward table center
+    # upper slingshots just below the two outer bumpers - kick toward centre
     dict(pos=(150.0, 282.0), r=11.0, kick=SLING_KICK_L),
     dict(pos=(330.0, 282.0), r=11.0, kick=SLING_KICK_R),
-    # guard posts at the outlane mouths - protect the corners
+    # guard posts at the outlane mouths: wall-huggers get bounced back into
+    # play instead of silently draining - the centre gap is the real risk
     dict(pos=(44.0, 314.0), r=12.0, kick=(0.80, -0.60)),
     dict(pos=(436.0, 314.0), r=12.0, kick=(-0.80, -0.60)),
 ]
@@ -137,7 +138,7 @@ class Table:
             for b in BUMPERS]
         self.slings = [
             dict(x=s["pos"][0], y=s["pos"][1], r=s["r"], cool=0, flash=0,
-                 kick=s["kick"], guard=(i >= 2))
+                 kick=s["kick"])
             for i, s in enumerate(SLINGS)]
         self.stars = [dict(x=x, y=y, r=13.0, lit=False, cool=0, pulse=0)
                       for x, y in STARS]