feat(sloppak): expose full-mix original_audio alongside stems (#583)

Lets a .sloppak ship the single pre-separation full mixdown next to its
per-instrument stems, so the player can use the pristine original when nothing
is isolated (demucs recombination is lossy) and switch to separated stems only
when a slider drops below unity.

- lib/sloppak.py::load_song parses the optional manifest `original_audio:` key
  into a new LoadedSloppak.original_audio field, with the same path-traversal
  guard + permissive "missing → disabled" posture as the drum_tab loader.
- The highway WS song_info frame additively carries original_audio_url (served
  by the existing /api/sloppak/{filename}/file/{rel_path} endpoint, None for
  stems-only packs), has_original_audio, and has_stems.
- A stem-less, full-mix-only sloppak now sets audio_url to the full mix (plays
  natively) instead of emitting audio_error.

Message shape stays a stable contract — all additions are purely additive.
Tests: tests/test_sloppak_original_audio_load.py (6 passing).

Closes #580

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/sloppak.py

@@ -367,6 +367,14 @@ class LoadedSloppak:
     # song.arrangements (not to manifest["arrangements"]) — skipped entries are
     # absent so indexing by song.arrangements index is safe.
     arrangement_ids: list[str | None] = field(default_factory=list)
+    # Manifest-relative path to the single full-mix audio file, taken from the
+    # manifest `original_audio:` key (e.g. "original/full.ogg"). This is the
+    # pre-separation mixdown that exists alongside the per-instrument `stems`.
+    # None when the key is absent, points outside source_dir, or the file is
+    # missing on disk. Served to the front-end via the highway WS as
+    # `original_audio_url`; the stems plugin uses it to play the untouched mix
+    # when every stem slider is at unity (and the separate stems otherwise).
+    original_audio: str | None = None
 
 
 def load_song(
@@ -808,6 +816,29 @@ def load_song(
                     }
 
     _fpv = manifest.get("feedpak_version")
+    # Optional full-mix audio — manifest `original_audio:` key. The single
+    # pre-separation mixdown that ships alongside the per-instrument stems.
+    # Same permissive, path-traversal-guarded posture as drum_tab above: a
+    # missing/escaping/absent file simply leaves the full mix unavailable (the
+    # player falls back to the separate stems) rather than aborting the load.
+    # We store the manifest-relative string so server.py can build its URL the
+    # same way it builds stem URLs (via the /api/sloppak/.../file/ endpoint).
+    original_audio_data: str | None = None
+    original_audio_rel = manifest.get("original_audio")
+    if isinstance(original_audio_rel, str) and original_audio_rel.strip():
+        rel = original_audio_rel.strip()
+        try:
+            oa_path = (source_dir / rel).resolve()
+            oa_path.relative_to(source_dir.resolve())
+        except ValueError:
+            log.warning("sloppak: original_audio path %r escapes source_dir — skipped", rel)
+            oa_path = None
+        except OSError as e:
+            log.warning("sloppak: original_audio path resolution failed (%s) — skipped", e)
+            oa_path = None
+        if oa_path is not None and oa_path.is_file():
+            original_audio_data = rel
+
     return LoadedSloppak(
         song=song,
         stems=stems,
@@ -821,6 +852,7 @@ def load_song(
         keys=keys_data,
         notation_by_id=notation_by_id_data,
         arrangement_ids=arrangement_ids_acc,
+        original_audio=original_audio_data,
     )