Problem: every FileReader owned its std::fs::File for its whole
lifetime, so each open media source pinned one descriptor. A project
with hundreds of sources hit the soft limit (256 on macOS, 1024 on
Linux) and the remaining media failed to open or showed as offline.
Fix: FileReader keeps its path and borrows the file from a shared LRU
pool of at most MAX_OPEN_FILES (128) open files. Past the cap the least
recently read file is closed and its reader reopens it on the next
read; a file that is now shorter than when it was opened is refused
instead of being read as the old one. Dropping a reader closes its file.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
