Implement soft artnet sync for LEDs

[SpComb]

Implement best-effort LEDs update synchronization for multiple Art-NET universes without explicit ArtSync frames (hard sync) from the sender. This attempts to match the LEDs update rate to the implicit Art-NET refresh rate, avoiding output tearing and reducing the number of LED updates for smoother results. This is particularly significant with larger numbers of LEDs where the maximum update rate is closer to the incoming Art-NET refresh rate.

When artnet_sync_timeout is configured, LED output updates are buffered until any of:

• An explicit Art-Sync frame is received
• All Art-NET universes have been buffered (full sync)
• More than artnet_sync_timeout ms have passed since the first buffered Art-NET universe was received (timeout sync)
• A second Art-NET frame was received for a buffered Art-NET universe (missed sync)

Using leds stats for testing: With an appropriate artnet_sync_timeout value matched to the expected refresh rate, this should ideally result in a leds task update rate that exactly matches the Art-NET refresh rate with zero sync misses/timeouts. However, if Art-NET frames are being dropped, or the sender is only sending Art-NET frames for some of the output universes, then the the sync missed/timeout should kick in and keep the output updating.