Pass per-stem name/description through to the stems payloads (#1013)

feedpak 1.16.0 (spec §5.3) added two OPTIONAL presentational fields to a
stems[] entry: `name` (display label, Readers fall back to the id) and
`description` (free text). The server dropped both while normalizing
manifest stems, so no client could ever display them.

Pass them through at the one place stem descriptors are built
(sloppak.load_song) and let both payload builders — the WS `ready` stems
list and the REST `/api/song/{f}?stems=1` preload list, which are pinned
against each other by test — carry them forward. Omit-when-absent, so a
stem without the fields does not grow null keys; non-string or blank
values are dropped rather than surfaced.

No behaviour change for existing packs or clients: the fields are
additive and every consumer that reads {id,url,default} keeps working
unchanged. The stems plugin / stem mixer display work lands separately.

Signed-off-by: topkoa <topkoa@gmail.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/routers/ws_highway.py

@@ -368,7 +368,9 @@ async def highway_ws(websocket: WebSocket, filename: str, arrangement: int = -1,
             q_fn = quote(filename, safe="")
             for s in loaded_slop.stems:
                 url = f"/api/sloppak/{q_fn}/file/{quote(s['file'])}"
-                stems_payload.append({"id": s["id"], "url": url, "default": s["default"]})
+                stems_payload.append(
+                    {"id": s["id"], "url": url, "default": s["default"],
+                     **{k: s[k] for k in ("name", "description") if k in s}})
             # Full-mix URL (served by the same /api/sloppak/.../file/ endpoint).
             if loaded_slop is not None and loaded_slop.full_mix:
                 full_mix_url = (