docs(downloads): mark UR-055/056 Done; fix colliding UT ids

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.

src/lib/utils/formatBytes.test.ts

@@ -1,7 +1,7 @@
 import { describe, it, expect } from "vitest";
 import { formatBytes } from "./formatBytes";
 
-// TRACES: UR-056 | DR-085 | UT-050
+// TRACES: UR-056 | DR-085 | UT-071
 describe("formatBytes", () => {
   it("renders zero and non-positive as '0 B'", () => {
     expect(formatBytes(0)).toBe("0 B");