fix(sloppak): the full mix is a stem — drop the invented original_audio key (#933)

Core read, served, and depended on `original_audio:` — a top-level manifest key
this repo invented in #583 that the feedpak spec never defined. The format
already had a home for the pre-separation mixdown: it is a stem. feedpak 1.15.0
(feedpak-spec#53) RESERVES the id `full` for it, so read it from there.

The key existed to work around a bug in our own reader. The packer's comment
said so plainly: "we must NOT list the full mix as a playable stem — the player
sums every entry in `stems` and does not gate playback on `default`, so a listed
full mix plays on top of the stems". Faced with a reader that would double the
song, the packer put the mixdown outside `stems` and invented a key to point at
it. The fix belongs in the reader, and that is what this is.

load_song() now partitions the stem list: `full` comes out as
LoadedSloppak.full_mix, the instruments stay in .stems. Nothing that sums stems
or draws one fader per stem can see the mixdown, so retaining it is safe — which
is what lets the packer put it where the format says it goes.

- ws_highway: `song_info` gains full_mix_url / has_full_mix. The old
  original_audio_url / has_original_audio remain as deprecated aliases for one
  release so an older stems plugin keeps working (#945).
- `stems` on the wire, and stem_ids / stem_count in the library index, are now
  INSTRUMENT stems only — a separated pack that retains its mixdown no longer
  advertises a bogus "full" chip or an inflated stem count.
- enrichment: fingerprint against the mixdown wherever it lives. This widens
  coverage — _song_audio_file() previously returned None for any pack without
  the invented key, so fingerprinting silently did nothing for nearly every pack.
- sloppak: `original_audio:` is still READ as a deprecated fallback, because
  every pack in the wild carries it and would otherwise lose its pristine mix.
  tools/migrate_full_mix_stem.py rewrites those packs into the spec shape
  (original/full.ogg -> stems/full.ogg, add the `full` stem at default:off, drop
  the key); the fallback and the aliases die with #945.

The spec gate keeps the debt honest: the grandfather entry now tracks #945, and
the gate fails if it goes stale.

Verified: spec gate OK (4/4, incl. ingesting the spec's new example pack that
retains `full`); 2493 python tests, 995 js tests; migrator round-tripped over
real packs from the library and the results pass the spec's reference validator.

static/highway.js

@@ -1908,17 +1908,21 @@ function createHighway() {
                                     // never routable.
                                     const isAudioUrl = msg.audio_url.startsWith('/audio/');
                                     // "Full mix" covers BOTH single-mix pack shapes:
-                                    //  - stem-less packs (original_audio: in the manifest,
-                                    //    audio_url == original_audio_url), and
-                                    //  - single-stem packs (stems: [full.ogg] only) — the server
-                                    //    puts the full mix in the stems list, has_original_audio
-                                    //    is false, and audio_url points at the one stem. With one
-                                    //    stem there is no per-stem mix to preserve, so routing it
-                                    //    natively loses nothing. Real multi-stem (>1) stays out
-                                    //    until Phase 2.
+                                    //  - single-stem packs (stems: [full.ogg] only) — the pack's
+                                    //    one stem IS its mixdown, so the server leaves it in the
+                                    //    stems list, has_full_mix is false, and audio_url points
+                                    //    at that one stem; and
+                                    //  - legacy stem-less packs, whose mixdown sits outside stems
+                                    //    behind the deprecated original_audio: key, so has_stems
+                                    //    is false and audio_url == full_mix_url.
+                                    // Either way there is one audible source and no per-stem mix
+                                    // to preserve, so routing it natively loses nothing. A pack
+                                    // that retains its `full` stem ALONGSIDE separated stems is
+                                    // multi-stem (has_full_mix && has_stems) and stays out until
+                                    // Phase 2 — routing it natively would drop the mixer.
                                     const isFeedpakFullMix = !isAudioUrl
                                         && msg.audio_url.startsWith('/api/sloppak/')
-                                        && ((!!msg.has_original_audio && !msg.has_stems)
+                                        && ((!!msg.has_full_mix && !msg.has_stems)
                                             || (msg.stems || []).length === 1);
                                     // Record the loaded song's audio so app.js can re-route it
                                     // between the HTML5 and JUCE paths if the audio engine is
@@ -1943,7 +1947,7 @@ function createHighway() {
                                         'isFeedpakFullMix=', isFeedpakFullMix,
                                         'has_stems=', !!msg.has_stems,
                                         'stems=', (msg.stems || []).length,
-                                        'has_original_audio=', !!msg.has_original_audio,
+                                        'has_full_mix=', !!msg.has_full_mix,
                                         'format=', msg.format,
                                         'alreadyLoaded=', alreadyLoaded,
                                         'juceApi=', !!window.feedBackDesktop?.audio);