fix: car now drives - collision groups, rigid hubs, pad spawn, anticheat grace

Four independent bugs kept the car immobile at the world origin:

1. Wheels collided with their own chassis. Wheel inner edge sits at 2.80
   vs body edge 3.00, and wheel top -0.20 vs body bottom -0.70. The
   overlap is intentional styling, but in one collision group the wheels
   ground against the body: motors spun ~1 rad/s against 20,000 torque.
   Wheels now use a CarWheel group that ignores CarChassis.

2. Suspension exploded on spawn. PrismaticConstraint+SpringConstraint
   between a 0.6-stud hub and the ballasted body was unstable enough to
   fling knuckles ~10,000,000 studs out, past FallenPartsDestroyHeight,
   deleting the wheels. Replaced with a rigid welded hub mount.

3. Car spawned at world origin, not the pad. MapBuilder made a SpawnPad
   part but no SpawnLocation, so characters spawned at (0,0,0) and Sit()
   dragged the car to them. Added a real SpawnLocation and the model is
   now pivoted after parenting, with the character teleported first.

4. AntiCheat snapped that legitimate spawn teleport back. An ~8,500-stud
   server move reads as a teleport hack. Added AntiCheat.grant() so
   server-initiated moves are exempt for a short grace window.

Verified in Studio: 881 studs in 6s, 496 studs/s top speed, 152 deg
steering, braking 33 -> 0 studs/s, chassis stays upright.

src/server/MapBuilder.luau

@@ -333,6 +333,21 @@ local function buildPA(map: Folder)
 	})
 	pad.Parent = pa
 
+	-- Roblox drops the character at (0,0,0) unless a real SpawnLocation exists,
+	-- and CarManager spawns the car onto the character -- so without this the
+	-- player starts ~8,600 studs off-circuit regardless of where SpawnPad is.
+	-- Neutral + non-collidable so it never becomes an obstacle on the pad.
+	local spawnLoc = Instance.new("SpawnLocation")
+	spawnLoc.Name = "PASpawn"
+	spawnLoc.Size = Vector3.new(24, 1, 24)
+	spawnLoc.CFrame = CFrame.new(c + Vector3.new(0, 2, 0))
+	spawnLoc.Transparency = 1
+	spawnLoc.CanCollide = false
+	spawnLoc.Anchored = true
+	spawnLoc.Neutral = true
+	spawnLoc.Duration = 0
+	spawnLoc.Parent = pa
+
 	makeSign("大黒PA  DAIKOKU PA", CFrame.new(c + Vector3.new(0, 26, -R + 10)), pa)
 end