Plexamp Lite 1.13: read ahead the next two tracks, keep loaded roots

Read-ahead keeps the next two tracks on the SD card (one without it) and drops files
that are no longer coming up. Trusted roots are cached once read, and a failed read
is retried instead of leaving every server untrusted until a restart.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -61,8 +61,8 @@ clock has been reset to before 2024 the certificate date check is skipped rather
   (default 192 kbps) caps both while the sound goes to a Bluetooth speaker: on the Mini, Wi-Fi and
   Bluetooth share one radio, and a lighter stream keeps the speaker from stuttering.
 * Seeking in a transcoded track restarts the transcode at the new position.
-* **Read-ahead:** while a track plays, the next one is fetched whole in the background and then
-  played from the file. Android 2.x's player keeps only a few seconds of stream buffer, so on weak
+* **Read-ahead:** while a track plays, the next two in the queue are fetched whole in the background
+  (one at a time) and then played from the file; without an SD card only the next one is kept. Android 2.x's player keeps only a few seconds of stream buffer, so on weak
   Wi-Fi shared with a Bluetooth speaker it used to go silent for seconds at a time.
 * If a stream fails, Plexamp Lite re-picks the connection and resumes; if it fails again it downloads
   the whole track first and plays from the SD card. "Download tracks before playing" forces that