Auto-detect time signature from SPP deltas (bar boundaries)

src/midi_transport.cpp

@@ -5,6 +5,7 @@
 volatile uint32_t midi_tick_count = 0;
 volatile uint16_t last_spp_position = 0;
 volatile bool spp_valid = false;
+extern volatile uint8_t beats_per_bar;
 
 static Adafruit_USBD_MIDI usb_midi;
 
@@ -80,9 +81,22 @@ void UsbMidiTransport::update() {
                 }
             } else if (cin == 0x03 && packet[1] == 0xF2) {
                 // Song Position Pointer
+                uint16_t prev_spp = last_spp_position;
                 last_spp_position = (packet[3] << 7) | packet[2];
                 spp_valid = true;
                 Serial.printf("[CLK] SPP=%d\n", last_spp_position);
+
+                // Auto-detect time signature from SPP delta (sent at bar boundaries)
+                if (prev_spp > 0 && last_spp_position > prev_spp) {
+                    uint16_t delta = last_spp_position - prev_spp;
+                    if (delta >= 8 && delta <= 64 && delta % 4 == 0) {
+                        uint8_t detected = delta / 4;
+                        if (detected >= 2 && detected <= 16 && detected != beats_per_bar) {
+                            beats_per_bar = detected;
+                            Serial.printf("[CLK] Auto-detected %d/4 time from SPP delta=%d\n", detected, delta);
+                        }
+                    }
+                }
             } else {
                 MidiEvent event;
                 parse_midi_packet(packet, 4, event);