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/AntiCheat.luau

@@ -25,6 +25,19 @@ type Track = { lastPos: Vector3, lastValid: CFrame }
 local tracked: { [Player]: Track } = {}
 local SAMPLE_INTERVAL = 0.5
 
+-- Server-initiated teleports (car spawn onto the pad, respawn, mode resets)
+-- legitimately move a player thousands of studs in one frame, which trips the
+-- speed check and snaps them straight back -- dragging their car with them.
+-- Callers announce those moves via AntiCheat.grant() so the next sample is
+-- accepted as the new baseline instead of treated as a hack.
+local graceUntil: { [Player]: number } = {}
+local GRACE_SECONDS = 2
+
+--- Exempt a player from the speed check briefly after a legitimate teleport.
+function AntiCheat.grant(player: Player, seconds: number?)
+	graceUntil[player] = os.clock() + (seconds or GRACE_SECONDS)
+end
+
 local function hrpOf(player: Player): BasePart?
 	local char = player.Character
 	return char and char:FindFirstChild("HumanoidRootPart") :: BasePart?
@@ -66,7 +79,8 @@ function AntiCheat.init()
 			local horizontal = Vector3.new(pos.X - track.lastPos.X, 0, pos.Z - track.lastPos.Z).Magnitude
 			local hSpeed = horizontal / interval
 
-			if hSpeed > GameConfig.MaxPlausibleSpeed then
+			local inGrace = (graceUntil[player] or 0) > os.clock()
+			if hSpeed > GameConfig.MaxPlausibleSpeed and not inGrace then
 				-- Snap back: likely a speed/teleport hack.
 				hrp.CFrame = track.lastValid
 				hrp.AssemblyLinearVelocity = Vector3.zero