[ao-david]

Stacked on #171.

• Pin NetSender HTTP work, the OTA listener, lwIP, the Ethernet receive task, and the FreeRTOS timer service to core 0. Audio runs on core 1 at priority 10.
• Protect shared device variables with a mutex and make the reload request atomic.
• Download new audio while the old file keeps playing, publish the new URL, and let the audio task perform the switch itself.
• Persist changed variables only from the audio task between tracks, keeping SD writes out of both polling and active playback.
• Retry transient SD read errors instead of treating them as EOF, which previously restarted the track from the beginning.
• Enlarge the I2S DMA ring to 32 x 512 frames (about 341 ms at 48 kHz) and raise task stacks where needed.
• Keep poll/vars progress logs at debug level to avoid SD log writes during polling.
• Document the playback isolation design.