The browsable Downloaded library + Transfers split + on-disk usage
(f25deba, plus today's grouping/perf fixes) fully implement UR-055 and
UR-056, but the requirements doc still listed them and DR-081..085 as
Planned. Flip to Done.
Also fix UT-id collisions: the downloaded-browse and formatBytes tests
reused UT-046..050 (already assigned to smart-cache/playlist tests in the
matrix). Reassign to UT-071..078 and register them in §4, including the
new music/TV container-rollup and orphan-leaf regression tests.