docs: scaffold polisher-control foundation

firmware/teensy/src/main.cpp

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+#include <Arduino.h>
+#include "polisher_protocol.h"
+#include "polisher_telemetry.h"
+
+static constexpr uint32_t HEARTBEAT_LED_MS = 500;
+uint32_t lastLedToggleMs = 0;
+bool ledState = false;
+
+void setup() {
+    pinMode(LED_BUILTIN, OUTPUT);
+    Serial.begin(115200);
+    // TODO: initialize protocol parser, CAN, encoders, drive I/O, watchdogs.
+}
+
+void loop() {
+    const uint32_t now = millis();
+    if (now - lastLedToggleMs >= HEARTBEAT_LED_MS) {
+        lastLedToggleMs = now;
+        ledState = !ledState;
+        digitalWrite(LED_BUILTIN, ledState ? HIGH : LOW);
+    }
+
+    // TODO v1 loop order:
+    // 1. Read host frames and validate CRC/version.
+    // 2. Read KWR75B-CAN and encoder inputs.
+    // 3. Update state/safety/watchdogs.
+    // 4. Compute setpoint ramp and force PID.
+    // 5. Apply actuator/drive outputs.
+    // 6. Emit telemetry >=100 Hz and events on transitions/faults.
+}