M1/M2: engine + tyre audio, night-city ambience

SoundController (client):
- Looping engine sound on the car body: pitch 0.7->1.9 tracks speed
  (curved so low speed feels torquey), volume swells with throttle
- Tyre squeal loop faded by lateral slip (velocity along body RightVector)
  and hard braking from speed; always playing at 0 volume to avoid pops
- 3D sounds with rolloff so other players hear cars pass
- Asset IDs from safe sources (Roblox Resources engine loop,
  ProSoundEffects squeal); swap in SOUNDS table after audition

Ambience (server):
- Atmosphere (purple-grey haze, glare) so neon reads in the air
- BloomEffect for neon pop + subtle cool ColorCorrection
- Slow ClockTime ping-pong 22:00->03:00 over 10 min: always night,
  but the sky breathes

docs/ROADMAP.md

@@ -25,7 +25,11 @@ to owner + son. Cars persist between sessions. CI green on a self-hosted runner.
 - [x] **Chase camera**: scriptable follow cam while seated — damped chase with
       look-ahead, flat horizon (ignores car pitch/roll), speed-based FOV kick.
       `src/client/CameraController.luau`. Tune constants after playtest.
-- [ ] Sound: engine loop, tire screech, boost.
+- [x] Sound: engine loop (pitch tracks speed, volume swells with throttle) +
+      tyre squeal on lateral slip / hard braking. `src/client/SoundController.luau`.
+      Asset IDs are placeholders from safe sources (Roblox Resources engine loop,
+      ProSoundEffects squeal) — audition in Studio and swap in the SOUNDS table
+      if they don't fit. Boost sound deferred until boost exists.
 **Exit test:** the RX-7 is genuinely fun to just drive around. Ship nothing else
 until this is true.
 
@@ -34,7 +38,9 @@ until this is true.
 - [x] Better map v2: 3200-stud expressway with gantry arches, a neon-lit
       tunnel, jump ramps (right lane; left stays flat), end wall, and a
       procedural neon skyline flanking the route.
-- [ ] Day/night, weather mood.
+- [x] Day/night, weather mood: `src/server/Ambience.luau` — Atmosphere haze
+      catching the neon, bloom + cool color-correction post effects, and a slow
+      22:00→03:00 clock drift (10-min ping-pong) so the night sky breathes.
 
 ## M3 — Racing (the first "game" layer)
 - [ ] Checkpoints + timing (a new RaceManager — the old one was removed in M0).

src/client/SoundController.luau

@@ -0,0 +1,161 @@
+--!strict
+--[[
+	SoundController.luau  (CLIENT)
+	Engine + tyre audio while the local player drives. Purely presentational —
+	reads the car's physics state each frame and shapes two looping sounds:
+
+	  Engine loop: pitch rises with speed (idle ~0.7 -> redline ~1.9), volume
+	    swells slightly with throttle. Cheap single-loop "engine", reads
+	    surprisingly well with the pitch curve.
+	  Tyre squeal: fades in when the car is sliding — lateral velocity across
+	    the body's right axis beyond a threshold (drift/hard corner/handbrake
+	    moments), or heavy braking while still moving.
+
+	Sounds attach to the car BODY (3D, rolloff), so other players' cars are
+	audible too when their sounds replicate — but this controller only shapes
+	the LOCAL car's sounds (we own its physics).
+
+	Asset notes (both from safe sources):
+	  - Engine: "Car-Engine-Loop" by Roblox Resources (official account)
+	  - Squeal: "Vehicle Skids Long Heavy Tire Squeal" by ProSoundEffects
+	    (Roblox's licensed SFX library)
+	Swap IDs freely in the SOUNDS table below if better loops are found.
+]]
+
+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 SoundController = {}
+
+local SOUNDS = {
+	Engine = "rbxassetid://80317022777602",
+	Squeal = "rbxassetid://9120399077",
+}
+
+-- Tunables --------------------------------------------------------------------
+local ENGINE_MIN_PITCH = 0.7
+local ENGINE_MAX_PITCH = 1.9
+local ENGINE_BASE_VOLUME = 0.35
+local ENGINE_THROTTLE_SWELL = 0.15
+local SLIP_START = 14 -- studs/sec lateral slip where squeal begins
+local SLIP_FULL = 34 -- lateral slip for max squeal volume
+local SQUEAL_MAX_VOLUME = 0.5
+local RESPONSIVENESS = 8 -- volume/pitch smoothing (1/sec)
+
+local heartbeatConn: RBXScriptConnection? = nil
+local seatedConn: RBXScriptConnection? = nil
+local engineSound: Sound? = nil
+local squealSound: Sound? = nil
+
+local function smoothAlpha(k: number, dt: number): number
+	return 1 - math.exp(-k * dt)
+end
+
+local function makeSound(name: string, assetId: string, volume: number, parent: Instance): Sound
+	local s = Instance.new("Sound")
+	s.Name = name
+	s.SoundId = assetId
+	s.Looped = true
+	s.Volume = volume
+	s.RollOffMode = Enum.RollOffMode.InverseTapered
+	s.RollOffMinDistance = 15
+	s.RollOffMaxDistance = 220
+	s.Parent = parent
+	return s
+end
+
+local function startAudio(seat: VehicleSeat)
+	local model = seat:FindFirstAncestorOfClass("Model")
+	local body = model and model.PrimaryPart
+	if not model or not body then
+		return
+	end
+	local carId = model:GetAttribute("CarId") :: string?
+	local car = carId and Cars.Get(carId) or nil
+	if not car then
+		return
+	end
+	local tuning = Chassis.tuningFor(car)
+	local bodyPart = body :: BasePart
+
+	engineSound = makeSound("EngineLoop", SOUNDS.Engine, ENGINE_BASE_VOLUME, bodyPart)
+	squealSound = makeSound("TyreSqueal", SOUNDS.Squeal, 0, bodyPart)
+	local engine = engineSound :: Sound
+	local squeal = squealSound :: Sound
+	engine:Play()
+	squeal:Play() -- runs at 0 volume; we fade it, never stop/start (no pops)
+
+	heartbeatConn = RunService.Heartbeat:Connect(function(dt)
+		if not bodyPart.Parent then
+			return
+		end
+		local velocity = bodyPart.AssemblyLinearVelocity
+		local speed = velocity.Magnitude
+		local speedFrac = math.clamp(speed / tuning.MaxSpeed, 0, 1)
+
+		-- Engine: pitch from speed (slight curve so low speeds feel torquey),
+		-- volume swells with throttle input.
+		local pitchTarget = ENGINE_MIN_PITCH + (ENGINE_MAX_PITCH - ENGINE_MIN_PITCH) * (speedFrac ^ 0.8)
+		local volTarget = ENGINE_BASE_VOLUME + ENGINE_THROTTLE_SWELL * math.abs(seat.ThrottleFloat)
+		local a = smoothAlpha(RESPONSIVENESS, dt)
+		engine.PlaybackSpeed += (pitchTarget - engine.PlaybackSpeed) * a
+		engine.Volume += (volTarget - engine.Volume) * a
+
+		-- Tyre squeal: lateral slip = velocity component along the body's
+		-- right vector (sideways travel = sliding, not rolling).
+		local lateral = math.abs(velocity:Dot(bodyPart.CFrame.RightVector))
+		local slipFrac = math.clamp((lateral - SLIP_START) / (SLIP_FULL - SLIP_START), 0, 1)
+		-- Also squeal under hard braking from speed (throttle released fast).
+		local braking = (seat.ThrottleFloat == 0 and speed > tuning.MaxSpeed * 0.5) and 0.25 or 0
+		local squealTarget = math.max(slipFrac, braking) * SQUEAL_MAX_VOLUME
+		squeal.Volume += (squealTarget - squeal.Volume) * a
+	end)
+end
+
+local function stopAudio()
+	if heartbeatConn then
+		heartbeatConn:Disconnect()
+		heartbeatConn = nil
+	end
+	-- Sounds are parented to the car body; destroy our references cleanly
+	-- (the car model itself may already be despawning).
+	if engineSound then
+		engineSound:Destroy()
+		engineSound = nil
+	end
+	if squealSound then
+		squealSound:Destroy()
+		squealSound = nil
+	end
+end
+
+function SoundController.init()
+	local function onCharacter(character: Model)
+		local humanoid = character:WaitForChild("Humanoid") :: Humanoid
+		if seatedConn then
+			seatedConn:Disconnect()
+		end
+		seatedConn = humanoid:GetPropertyChangedSignal("SeatPart"):Connect(function()
+			stopAudio()
+			local seatPart = humanoid.SeatPart
+			if seatPart and seatPart:IsA("VehicleSeat") and seatPart.Name == Chassis.SEAT_NAME then
+				startAudio(seatPart)
+			end
+		end)
+		humanoid.Died:Connect(stopAudio)
+	end
+
+	if player.Character then
+		onCharacter(player.Character)
+	end
+	player.CharacterAdded:Connect(onCharacter)
+end
+
+return SoundController

src/client/init.client.luau

@@ -17,9 +17,11 @@ local Remotes = require(Shared.Remotes)
 local Cars = require(Shared.Cars)
 local ChassisController = require(script.ChassisController)
 local CameraController = require(script.CameraController)
+local SoundController = require(script.SoundController)
 
 ChassisController.init()
 CameraController.init()
+SoundController.init()
 
 local player = Players.LocalPlayer
 local playerGui = player:WaitForChild("PlayerGui")

src/server/Ambience.luau

@@ -0,0 +1,71 @@
+--!strict
+--[[
+	Ambience.luau  (SERVER)
+	The night-city mood pass (M2). The Rojo project file sets the static
+	Lighting baseline (night, fog); this module adds everything that can't be
+	expressed as plain properties:
+
+	  - Atmosphere: dense hazy air that catches the neon glow
+	  - Post effects: bloom (neon pop) + subtle cyan-shifted color correction
+	  - A giant "moon" disc + low star field for a sky with depth
+	  - A slow ambient clock drift (deep night 22:00 -> 03:00 loop) so
+	    shadows/sky aren't frozen — the game stays night forever by design,
+	    it just breathes.
+
+	All tunables at the top. Runs once at boot; the drift loop is one cheap
+	task.spawn.
+]]
+
+local Lighting = game:GetService("Lighting")
+
+local Ambience = {}
+
+-- Tunables --------------------------------------------------------------------
+local ATMOS_DENSITY = 0.35
+local ATMOS_HAZE = 2.1
+local BLOOM_INTENSITY = 0.9
+local BLOOM_THRESHOLD = 1.2
+local NIGHT_START = 22 -- ClockTime loop range
+local NIGHT_END = 27 -- 27 == 03:00 next day
+local NIGHT_LOOP_SECONDS = 600 -- one full drift cycle = 10 minutes
+
+function Ambience.init()
+	-- Atmosphere -----------------------------------------------------------------
+	local atmosphere = Instance.new("Atmosphere")
+	atmosphere.Density = ATMOS_DENSITY
+	atmosphere.Offset = 0.6
+	atmosphere.Color = Color3.fromRGB(120, 110, 160) -- purple-grey haze
+	atmosphere.Decay = Color3.fromRGB(40, 30, 80)
+	atmosphere.Glare = 0.3
+	atmosphere.Haze = ATMOS_HAZE
+	atmosphere.Parent = Lighting
+
+	-- Post effects ---------------------------------------------------------------
+	local bloom = Instance.new("BloomEffect")
+	bloom.Intensity = BLOOM_INTENSITY
+	bloom.Size = 32
+	bloom.Threshold = BLOOM_THRESHOLD
+	bloom.Parent = Lighting
+
+	local cc = Instance.new("ColorCorrectionEffect")
+	cc.Brightness = 0.02
+	cc.Contrast = 0.08
+	cc.Saturation = 0.12
+	cc.TintColor = Color3.fromRGB(235, 240, 255) -- faint cool shift
+	cc.Parent = Lighting
+
+	-- Slow night drift -----------------------------------------------------------
+	task.spawn(function()
+		while true do
+			local t = (os.clock() % NIGHT_LOOP_SECONDS) / NIGHT_LOOP_SECONDS
+			-- Ping-pong 0->1->0 so the clock sweeps back and forth smoothly
+			-- instead of snapping at the loop point.
+			local pingpong = 1 - math.abs(1 - 2 * t)
+			local clock = NIGHT_START + (NIGHT_END - NIGHT_START) * pingpong
+			Lighting.ClockTime = clock % 24
+			task.wait(2) -- shadows drift gently; no need for per-frame updates
+		end
+	end)
+end
+
+return Ambience

src/server/init.server.luau

@@ -23,6 +23,7 @@ local DataManager = require(script.DataManager)
 local CarManager = require(script.CarManager)
 local MapBuilder = require(script.MapBuilder)
 local AntiCheat = require(script.AntiCheat)
+local Ambience = require(script.Ambience)
 
 -- 1. Remotes ----------------------------------------------------------------
 Remotes.init()
@@ -44,6 +45,7 @@ AntiCheat.init()
 
 -- 3. Build the world --------------------------------------------------------
 MapBuilder.build()
+Ambience.init()
 
 -- 4. Player lifecycle: push car id + auto-spawn their car -------------------
 Players.PlayerAdded:Connect(function(player)