Project UNION MP3 Player U3.22 - DCC diagnostic build

Based on U3.21. No changes to DCC decoding, DFPlayer playback, MQTT,
NVS storage or ProjectUNION libraries. Only transient diagnostic counters,
Serial Monitor messages, and a diagnostic card on the DCC page.

How to test with Michael:
1. Back up the configuration before updating.
2. Upload firmware using the existing working partition scheme.
3. Open DCC page, send an EXTERNAL accessory command, refresh (F5).
4. Report External received / Dispatched / Ignored, Last dispatch,
   DFPlayer ready, Playing, Track, Last command.
5. Repeat with the internal Test button and compare.
6. Optionally capture Serial Monitor at 115200 baud.

Counters reset after reboot. A displayed 'Dispatched' event means that
software called the playback action; it does NOT prove audio was produced.
