Real car meshes + physics-derived suspension + inPA/void fixes

Car meshes:
- CarManager: clone ReplicatedStorage/CarModels/<carId> as a visual Shell,
  weld to Body, hide placeholder blocks. Falls back to procedural CarBody
  when no mesh exists, so cars can be converted one at a time.
- Shell parts are Massless/CanCollide=false/CanQuery=false: the constraint
  chassis and driving feel are untouched. Verified 0 violating parts live.
- 5 catalogue cars mapped from the 67-model ServerStorage/CarLibrary import.

Suspension (ref docs/CAR_PHYSICS_REFERENCE.txt, devforum 3915628):
- BUG: old k=9000 gave 1.10 studs static sag against 0.90 studs total travel,
  i.e. the suspension sat permanently bottomed out and the spring did no work.
- Stiffness now solved from a sag target (70%% of half travel); damping now
  derived as a damping ratio c = zeta*2*sqrt(k*m) instead of an unrelated
  constant. Old pair worked out to zeta=0.31 (floaty); now 0.55-0.63.
- Measured after fix: ride height 1.97 studs, bounce 0.000, body roll 0.002.

Also: Circuit.inPA square test (PAFloor is 520x520, test was a r=260 circle,
corners falsely reported outside and spuriously armed runs), and void-fall
recovery now moves the character to the spawn pad BEFORE building the car.

src/server/MapBuilder.luau

@@ -323,6 +323,64 @@ local function buildPA(map: Folder)
 	})
 	ramp.Parent = pa
 
+	-- PERIMETER BARRIERS: the PA floor is a 520x520 slab floating ~46 studs up
+	-- with nothing at its edges. Endurance testing drove straight off the side
+	-- and fell past FallenPartsDestroyHeight (-500), destroying the car and
+	-- stranding the player seatless until the death/respawn cycle caught it.
+	-- Wall every edge, leaving a gap on the side the ramp exits through so the
+	-- route to the expressway stays open.
+	do
+		local rampDir = Vector3.new(road.X - c.X, 0, road.Z - c.Z)
+		-- Which edge does the ramp leave from? Whichever axis dominates.
+		local rampOnX = math.abs(rampDir.X) > math.abs(rampDir.Z)
+		local rampSign = rampOnX and math.sign(rampDir.X) or math.sign(rampDir.Z)
+		local GAP = 70 -- half-width of the opening (ramp is 46 wide)
+		local H, T = 14, 4
+
+		local function wall(name, size, pos)
+			local w = part({
+				Name = name,
+				Size = size,
+				CFrame = CFrame.new(pos),
+				Color = Color3.fromRGB(96, 96, 104),
+				Material = Enum.Material.Concrete,
+			})
+			w.Parent = pa
+		end
+
+		-- Four edges. On the ramp's edge, build two stubs flanking the opening.
+		local edges = {
+			{ onX = true, sign = 1 },
+			{ onX = true, sign = -1 },
+			{ onX = false, sign = 1 },
+			{ onX = false, sign = -1 },
+		}
+		for _, e in ipairs(edges) do
+			local isRampEdge = (e.onX == rampOnX) and (e.sign == rampSign)
+			-- Offset along the edge normal; span the full slab on the other axis.
+			local normal = e.onX and Vector3.new(e.sign * R, 0, 0) or Vector3.new(0, 0, e.sign * R)
+			local base = c + normal + Vector3.new(0, H / 2, 0)
+			local fullSize = e.onX and Vector3.new(T, H, R * 2) or Vector3.new(R * 2, H, T)
+			if not isRampEdge then
+				wall("PAWall", fullSize, base)
+			else
+				-- two stubs, each running from the corner in to the gap edge
+				local stubLen = R - GAP
+				if stubLen > 4 then
+					local half = stubLen / 2
+					local off = GAP + half
+					if e.onX then
+						wall("PAWall", Vector3.new(T, H, stubLen), base + Vector3.new(0, 0, off))
+						wall("PAWall", Vector3.new(T, H, stubLen), base + Vector3.new(0, 0, -off))
+					else
+						wall("PAWall", Vector3.new(stubLen, H, T), base + Vector3.new(off, 0, 0))
+						wall("PAWall", Vector3.new(stubLen, H, T), base + Vector3.new(-off, 0, 0))
+					end
+				end
+			end
+		end
+	end
+
 	local pad = part({
 		Name = "SpawnPad",
 		Size = Vector3.new(120, 2.4, 120),