feat: initial AC telemetry Lua plugin

- CSP Lua app that reads telemetry via ac.* API
- Sends JSON over UDP to configurable target (default 127.0.0.1:5005)
- Purpose-built serializer with zero GC pressure at 60Hz
- Pre-allocated buffer tables, no per-frame allocations
- Graceful fallback when ac global is absent (testing outside AC)
- .gitignore for .idea/

src/json.lua

@@ -0,0 +1,21 @@
+-- src/json.lua - Purpose-built telemetry serializer for ACTelemetry
+-- Writes directly to a string buffer without table allocation or double traversal.
+
+local json = {}
+
+function json.encode_telemetry(d)
+    local g = d.g_forces
+    local ws = d.wheel_slip
+    local st = d.suspension_travel
+    return string.format(
+        '{"timestamp":%d,"speed_kmh":%.1f,"rpm":%.0f,"gear":%d,"throttle":%.3f,"brake":%.3f,"steering":%.3f,"g_forces":{"x":%.3f,"y":%.3f,"z":%.3f},"wheel_slip":[%.3f,%.3f,%.3f,%.3f],"suspension_travel":[%.3f,%.3f,%.3f,%.3f],"lap_time":%.3f,"lap_count":%d}',
+        d.timestamp, d.speed_kmh, d.rpm, d.gear,
+        d.throttle, d.brake, d.steering,
+        g.x, g.y, g.z,
+        ws[1], ws[2], ws[3], ws[4],
+        st[1], st[2], st[3], st[4],
+        d.lap_time, d.lap_count
+    )
+end
+
+return json

src/telemetry.lua

@@ -0,0 +1,144 @@
+-- src/telemetry.lua - Telemetry reader and UDP sender for ACTelemetry
+-- Reads car/session state via the CSP Lua API and sends JSON over UDP at a configured rate.
+
+local config = require("config")
+local json = require("src.json")
+
+local telemetry = {}
+
+local socket = require("socket.udp")
+local udp = socket()
+
+local last_send_time = 0
+
+local ac_available = type(ac) == "table" and type(ac.getCarState) == "function"
+
+local buf_g_forces = { x = 0, y = 0, z = 0 }
+local buf_wheel_slip = { 0, 0, 0, 0 }
+local buf_suspension_travel = { 0, 0, 0, 0 }
+
+local buf = {
+    timestamp = 0,
+    speed_kmh = 0,
+    rpm = 0,
+    gear = 0,
+    throttle = 0,
+    brake = 0,
+    steering = 0,
+    g_forces = buf_g_forces,
+    wheel_slip = buf_wheel_slip,
+    suspension_travel = buf_suspension_travel,
+    lap_time = 0,
+    lap_count = 0,
+}
+
+function telemetry.collect()
+    if not ac_available then
+        buf.timestamp = os.time()
+        buf.speed_kmh = 0
+        buf.rpm = 0
+        buf.gear = 0
+        buf.throttle = 0
+        buf.brake = 0
+        buf.steering = 0
+        buf_g_forces.x = 0
+        buf_g_forces.y = 0
+        buf_g_forces.z = 0
+        buf_wheel_slip[1] = 0 buf_wheel_slip[2] = 0 buf_wheel_slip[3] = 0 buf_wheel_slip[4] = 0
+        buf_suspension_travel[1] = 0 buf_suspension_travel[2] = 0 buf_suspension_travel[3] = 0 buf_suspension_travel[4] = 0
+        buf.lap_time = 0
+        buf.lap_count = 0
+        return buf
+    end
+
+    local car = ac.getCarState(0)
+
+    buf.timestamp = os.time()
+    buf.speed_kmh = car.speedKmh or 0
+    buf.rpm = car.rpm or car.rpms or 0
+    buf.gear = car.gear or 0
+    buf.throttle = car.gas or 0
+    buf.brake = car.brake or 0
+    buf.steering = car.steer or 0
+
+    local gx, gy, gz = 0, 0, 0
+    if car.G then
+        gx = car.G.x or 0
+        gy = car.G.y or 0
+        gz = car.G.z or 0
+    end
+    buf_g_forces.x = gx
+    buf_g_forces.y = gy
+    buf_g_forces.z = gz
+
+    buf_wheel_slip[1] = 0 buf_wheel_slip[2] = 0 buf_wheel_slip[3] = 0 buf_wheel_slip[4] = 0
+    buf_suspension_travel[1] = 0 buf_suspension_travel[2] = 0 buf_suspension_travel[3] = 0 buf_suspension_travel[4] = 0
+
+    if car.wheels then
+        for i = 1, math.min(#car.wheels, 4) do
+            local w = car.wheels[i]
+            buf_wheel_slip[i] = w.slip or 0
+            buf_suspension_travel[i] = w.suspensionTravel or 0
+        end
+    elseif car.wheelSlips then
+        for i = 1, math.min(#car.wheelSlips, 4) do
+            buf_wheel_slip[i] = car.wheelSlips[i] or 0
+        end
+        if car.suspensionTravel then
+            for i = 1, math.min(#car.suspensionTravel, 4) do
+                buf_suspension_travel[i] = car.suspensionTravel[i] or 0
+            end
+        elseif car.wheelSuspensionTravel then
+            for i = 1, math.min(#car.wheelSuspensionTravel, 4) do
+                buf_suspension_travel[i] = car.wheelSuspensionTravel[i] or 0
+            end
+        end
+    end
+
+    local lap_time = 0
+    if car.lapTimeMs then
+        lap_time = car.lapTimeMs / 1000
+    elseif car.lapTime then
+        lap_time = car.lapTime
+    end
+    buf.lap_time = lap_time
+
+    local lap_count = 0
+    if car.lapCount then
+        lap_count = car.lapCount
+    elseif car.lapIndex then
+        lap_count = car.lapIndex
+    end
+    buf.lap_count = lap_count
+
+    return buf
+end
+
+local payload_buf = ""
+
+local function do_send()
+    udp:sendto(payload_buf, config.target_host, config.target_port)
+end
+
+function telemetry.send(data)
+    payload_buf = json.encode_telemetry(data)
+    local ok, err = pcall(do_send)
+    if not ok then
+        if ac_available then
+            ac.log("[ACTelemetry] UDP send failed: " .. tostring(err))
+        else
+            print("[ACTelemetry] UDP send failed: " .. tostring(err))
+        end
+    end
+end
+
+function telemetry.update(dt)
+    local now = os.clock()
+    if now - last_send_time >= config.send_interval then
+        last_send_time = now
+        local data = telemetry.collect()
+        telemetry.send(data)
+    end
+end
+
+return telemetry