fix(CarManager): repair car torn apart on spawn by client-owned character

Sit() dragged the seat's body assembly ~15,700 studs to the client-owned
character while the knuckle/wheel assemblies stayed at the spawn pad,
tearing the car apart; wheels then fell out of the world.

Server-side character:PivotTo() does not stick (client has authority) --
it holds one frame and snaps back, so the pre-seat teleport could not fix
this. Instead: record each part's offset from the body while the model is
intact, take server network ownership, let Sit() and the snap-back settle,
then reassemble every assembly around the body (forced level) and hand
ownership back to the driver.

Verified in Studio: max part separation 6.42 studs (exhaust, normal),
up-vector 1.00, driver seated, drove 98.6 studs at 41 studs/s intact.

src/server/CarManager.luau

@@ -350,14 +350,80 @@ function CarManager.spawn(player: Player, carId: string?)
 	if character then
 		local seat = model:FindFirstChild(Chassis.SEAT_NAME) :: VehicleSeat
 		if seat then
-			-- Teleport the character to the car before seating. Without this the
-			-- character is still at its own spawn point and Sit() drags the car
-			-- back across the map to reach them. Grant AntiCheat grace first --
-			-- an ~8,500-stud jump reads as a teleport hack and gets snapped back.
+			-- The character is CLIENT-owned, so a server-side PivotTo of it does
+			-- NOT stick: it holds for one frame and the client snaps it back to
+			-- where it was. Sit() then drags the seat's assembly across the map
+			-- to reach the character -- and because a constraint chassis keeps
+			-- the body, knuckles and wheels as SEPARATE assemblies, only the
+			-- body follows. The knuckles/wheels stay behind at the pad and the
+			-- car is torn apart (measured ~15,700 studs), later falling out of
+			-- the world.
+			--
+			-- Fix: don't fight the client. Record every part's offset from the
+			-- body while the model is still intact, let Sit() and the character
+			-- snap-back fully settle, then REASSEMBLE the car around wherever
+			-- the body actually ended up.
+			local primary = model.PrimaryPart
+			local offsets: { [BasePart]: CFrame } = {}
+			local movable: { BasePart } = {}
+			if primary then
+				local inverse = primary.CFrame:Inverse()
+				for _, desc in ipairs(model:GetDescendants()) do
+					if desc:IsA("BasePart") and not desc.Anchored then
+						offsets[desc] = inverse * desc.CFrame
+						table.insert(movable, desc)
+					end
+				end
+			end
+
 			AntiCheat.grant(player)
 			character:PivotTo(target * CFrame.new(0, 3, 0))
 			task.defer(function()
 				seat:Sit(character:FindFirstChildOfClass("Humanoid"))
+
+				if not primary then
+					return
+				end
+				-- Hold server authority while the rig is in pieces, otherwise
+				-- the client re-asserts the torn layout over our repair.
+				for _, part in ipairs(movable) do
+					pcall(function()
+						part:SetNetworkOwner(nil)
+					end)
+				end
+
+				task.wait(1.2) -- Sit() + client snap-back settle here
+
+				if not primary:IsDescendantOf(Workspace) then
+					return
+				end
+				-- Rebuild every assembly around the body, forced level so the
+				-- car never lands on its roof.
+				local position = primary.Position
+				local look = primary.CFrame.LookVector
+				local flat = Vector3.new(look.X, 0, look.Z)
+				if flat.Magnitude < 0.1 then
+					flat = Vector3.new(0, 0, -1)
+				end
+				local base = CFrame.lookAt(position, position + flat.Unit)
+				for _, part in ipairs(movable) do
+					if part:IsDescendantOf(Workspace) then
+						part.CFrame = base * offsets[part]
+						part.AssemblyLinearVelocity = Vector3.zero
+						part.AssemblyAngularVelocity = Vector3.zero
+					end
+				end
+
+				task.wait(0.3) -- let the solver accept the repaired rig
+
+				AntiCheat.grant(player)
+				for _, part in ipairs(movable) do
+					if part:IsDescendantOf(Workspace) then
+						pcall(function()
+							part:SetNetworkOwner(player)
+						end)
+					end
+				end
 			end)
 		end
 	end