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/sloppak.py

@@ -1114,11 +1114,19 @@ def load_song(
         sfile = str(s.get("file", ""))
         if not sid or not sfile:
             continue
-        stems.append({
+        entry = {
             "id": sid,
             "file": sfile,
             "default": stem_default_on(s.get("default", True)),
-        })
+        }
+        # Optional presentational fields (feedpak 1.16.0, spec §5.3). Omitted —
+        # not None — when absent, so payload builders can pass entries through
+        # without every stem growing null keys.
+        for key in ("name", "description"):
+            val = s.get(key)
+            if isinstance(val, str) and val.strip():
+                entry[key] = val
+        stems.append(entry)
 
     # The complete mixdown is a stem (spec §5.3), but it is not a *layer*: lift
     # it out so that no consumer of `stems` — the mixer, the library's stem