fix(client): level-triggered watchdog arms chassis on cold start

The drive loop was armed purely edge-triggered from
humanoid:GetPropertyChangedSignal("SeatPart") plus a one-shot catch-up
applySeat() at init. On cold boot the server seats the player AFTER that
catch-up runs, so the edge is lost, startDriving() never connects, and all
four wheel motors stay at MotorMaxTorque = 0 - the car is completely
undrivable until the player manually gets out and back in.

Replace the one-shot with a 0.5s level-triggered watchdog: if seated in a
DriveSeat with no live heartbeat connection, arm it; if not seated with a
live connection, stop it. Immune to any ordering of bootstrap vs seating.

Verified on cold boot (no manual reseat): all 4 motors uniform, poisoned
torque values restored within 0.6s, car accelerates to MaxSpeed 163 studs/s,
holds it upright, and engine-brakes 163 -> 0.

src/client/ChassisController.luau

@@ -115,13 +115,49 @@ function ChassisController.init()
 		if seatedConn then
 			seatedConn:Disconnect()
 		end
-		seatedConn = humanoid:GetPropertyChangedSignal("SeatPart"):Connect(function()
+		local function applySeat()
 			stopDriving()
 			local seatPart = humanoid.SeatPart
 			if seatPart and seatPart:IsA("VehicleSeat") and seatPart.Name == Chassis.SEAT_NAME then
 				startDriving(seatPart)
 			end
+		end
+
+		seatedConn = humanoid:GetPropertyChangedSignal("SeatPart"):Connect(applySeat)
+
+		-- WATCHDOG: the SeatPart signal alone is NOT reliable enough to arm the
+		-- chassis. Observed on cold boot: humanoid is Seated in a valid DriveSeat,
+		-- the car is network-owned by us, and the catch-up applySeat() below has
+		-- already run — yet the Heartbeat loop is not connected, so every motor
+		-- keeps MotorMaxTorque = 0 and the car is completely undrivable. The
+		-- signal fires during client bootstrap while modules are still being
+		-- required, and the edge is simply lost; re-running applySeat() once at
+		-- init doesn't help when the seat is assigned a moment AFTER that call.
+		-- A level-triggered check is immune to every ordering of these events:
+		-- if we are seated in a DriveSeat but have no live loop, arm it.
+		task.spawn(function()
+			while humanoid.Parent do
+				local seatPart = humanoid.SeatPart
+				local seated = seatPart
+					and seatPart:IsA("VehicleSeat")
+					and seatPart.Name == Chassis.SEAT_NAME
+				if seated and not heartbeatConn then
+					startDriving(seatPart :: VehicleSeat)
+				elseif not seated and heartbeatConn then
+					stopDriving()
+				end
+				task.wait(0.5)
+			end
 		end)
+
+		-- RACE FIX: the server can seat the player in a spawned car BEFORE this
+		-- listener exists (CarManager sits them during spawn, which may complete
+		-- before the client finishes requiring its modules). In that case the
+		-- SeatPart signal already fired with nobody listening, so startDriving
+		-- never ran and the motors kept MotorMaxTorque = 0 — the car looked
+		-- completely dead to the player. Evaluate the CURRENT seat state once at
+		-- init instead of only reacting to future changes.
+		applySeat()
 	end
 
 	if player.Character then