The desktop host had no Services::reader, so MediaPool::open_file read every
clip whole and kept its bytes for as long as the source stayed open: 2.9 GB
for twelve clips on a timeline (malloc_history, 15 allocations in open_file).
FsServices::reader now returns a FileReader that keeps the file open and
serves positional reads, so MP4/MOV, Matroska and the other reader openers
load the index and then single samples, as the web host already does.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
