M1: constraint chassis (springs, servo steering, motor wheels)

- New shared Chassis.luau: geometry constants + per-car tuning from stats
- CarManager.buildCarModel rebuilt: prismatic+spring suspension per corner,
  servo-hinge steering on front knuckles, motor-hinge driven wheels,
  hidden dense ballast for anti-roll; VehicleSeat kept as input device only
- New client ChassisController: drives motors/servos from seat
  ThrottleFloat/SteerFloat per Heartbeat; speed-sensitive steering lock;
  engine braking on throttle release
- Driver gets network ownership of every unanchored part (constraint
  assemblies are separate; missing one causes stuttery physics)
- Docs: MESH_SWAP.md rewritten for shell-on-chassis workflow; ROADMAP updated

src/client/ChassisController.luau

@@ -0,0 +1,133 @@
+--!strict
+--[[
+	ChassisController.luau  (CLIENT)
+	Drives the constraint chassis while the local player is in a DriveSeat.
+
+	Why client-side: the server gives the driver network ownership of the car,
+	so physics (and constraint targets) run authoritatively on THIS client —
+	that's what makes driving feel responsive instead of laggy. The server
+	still owns WHICH car you get (allowlist etc.); this module only turns
+	seat input into motor/servo targets.
+
+	Loop per Heartbeat while seated:
+	  Steer   -> front servo TargetAngle (speed-sensitive: less lock when fast)
+	  Throttle-> wheel motor AngularVelocity + MotorMaxTorque
+	  No input-> brake torque toward zero angular velocity
+]]
+
+local Players = game:GetService("Players")
+local RunService = game:GetService("RunService")
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+
+local Shared = ReplicatedStorage.Shared
+local Cars = require(Shared.Cars)
+local Chassis = require(Shared.Chassis)
+
+local player = Players.LocalPlayer
+
+local ChassisController = {}
+
+local heartbeatConn: RBXScriptConnection? = nil
+local seatedConn: RBXScriptConnection? = nil
+
+local function findCarModel(seat: VehicleSeat): Model?
+	local model = seat:FindFirstAncestorOfClass("Model")
+	if model and model:GetAttribute("CarId") ~= nil then
+		return model
+	end
+	return nil
+end
+
+local function collectConstraints(model: Model): ({ HingeConstraint }, { HingeConstraint })
+	local motors: { HingeConstraint } = {}
+	local servos: { HingeConstraint } = {}
+	for _, desc in ipairs(model:GetDescendants()) do
+		if desc:IsA("HingeConstraint") then
+			if desc.Name == Chassis.WHEEL_MOTOR_NAME then
+				table.insert(motors, desc)
+			elseif desc.Name == Chassis.STEER_SERVO_NAME then
+				table.insert(servos, desc)
+			end
+		end
+	end
+	return motors, servos
+end
+
+local function startDriving(seat: VehicleSeat)
+	local model = findCarModel(seat)
+	if not model then
+		return
+	end
+	local carId = model:GetAttribute("CarId") :: string
+	local car = Cars.Get(carId)
+	if not car then
+		return
+	end
+	local tuning = Chassis.tuningFor(car)
+	local motors, servos = collectConstraints(model)
+	local body = model.PrimaryPart
+	if #motors == 0 or not body then
+		return
+	end
+
+	heartbeatConn = RunService.Heartbeat:Connect(function()
+		local throttle = seat.ThrottleFloat
+		local steer = seat.SteerFloat
+
+		-- Steering: speed-sensitive — at top speed only ~40% of full lock,
+		-- so the car doesn't spin out from a tap of the key at 150 studs/s.
+		local speed = body.AssemblyLinearVelocity.Magnitude
+		local speedFrac = math.clamp(speed / tuning.MaxSpeed, 0, 1)
+		local lockScale = 1 - 0.6 * speedFrac
+		local targetAngle = Chassis.STEER_SIGN * steer * tuning.MaxSteerAngle * lockScale
+		for _, servo in ipairs(servos) do
+			servo.TargetAngle = targetAngle
+		end
+
+		-- Drive/brake:
+		if math.abs(throttle) > 0.05 then
+			local targetVel = Chassis.DRIVE_SIGN * throttle * tuning.MaxWheelAngularVelocity
+			for _, motor in ipairs(motors) do
+				motor.AngularVelocity = targetVel
+				motor.MotorMaxTorque = tuning.MotorMaxTorque
+			end
+		else
+			-- Engine braking: drive wheel speed toward zero with brake torque.
+			for _, motor in ipairs(motors) do
+				motor.AngularVelocity = 0
+				motor.MotorMaxTorque = tuning.BrakeTorque
+			end
+		end
+	end)
+end
+
+local function stopDriving()
+	if heartbeatConn then
+		heartbeatConn:Disconnect()
+		heartbeatConn = nil
+	end
+end
+
+function ChassisController.init()
+	-- SeatPart changes whenever the humanoid sits down or gets up.
+	local function onCharacter(character: Model)
+		local humanoid = character:WaitForChild("Humanoid") :: Humanoid
+		if seatedConn then
+			seatedConn:Disconnect()
+		end
+		seatedConn = humanoid:GetPropertyChangedSignal("SeatPart"):Connect(function()
+			stopDriving()
+			local seatPart = humanoid.SeatPart
+			if seatPart and seatPart:IsA("VehicleSeat") and seatPart.Name == Chassis.SEAT_NAME then
+				startDriving(seatPart)
+			end
+		end)
+	end
+
+	if player.Character then
+		onCharacter(player.Character)
+	end
+	player.CharacterAdded:Connect(onCharacter)
+end
+
+return ChassisController

src/client/init.client.luau

@@ -15,6 +15,9 @@ local ReplicatedStorage = game:GetService("ReplicatedStorage")
 local Shared = ReplicatedStorage.Shared
 local Remotes = require(Shared.Remotes)
 local Cars = require(Shared.Cars)
+local ChassisController = require(script.ChassisController)
+
+ChassisController.init()
 
 local player = Players.LocalPlayer
 local playerGui = player:WaitForChild("PlayerGui")