fix(hub): repair NUL-byte corruption in music-app.js, simplify grouping

Two spots in music-app.js held a literal NUL byte where a plain space
was intended - present since the file's very first commit (the
LazyTile key's template literal joining artist and album) and
reintroduced by the previous commit's sentinel key for the untagged
bucket. Root cause looks like a tool/transport artifact rather than
anything in the logic itself: git rendering the file as binary
("0 insertions/0 deletions" on a real content change) was the tell,
caught before it reached anyone actually running this code.

Fixed by rewriting the file clean and, while at it, removing the
sentinel-key approach that produced the second NUL entirely: the
untagged-tracks bucket is now a plain array on each artist entry,
appended as a synthetic album only at the end, rather than a Map key
that had to be guaranteed to never collide with a real folded album
name. No behavioural change from the previous commit's intent - same
grouping, same flat-mode rendering - just without the fragile
mechanism that broke.

Verified: zero NUL bytes anywhere in the file (checked the whole repo
for the same class of corruption - only genuine binaries, PNG/WASM,
have any), valid UTF-8, node --check passes, git diff renders as text
again. npm run sync-ui re-run. test_hook_ordering.py,
test_transport_contracts.py, test_locales.py: 31 passed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KBi7ALLGfwcjBXt57yNMcy

0 files changed, 0 insertions, 0 deletions