fix(sloppak): the full mix is a stem — drop the invented original_audio key (#946)

* 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.

* fix(migrate): discover directory-form packs instead of silently skipping them

iter_packs() searched only files, so a directory-form pack (`song.sloppak/`, the
authoring shape) was walked INTO and never yielded — silently missed by a run
that's meant to be exhaustive. Discover suffix-named directories too (yielded
whole, not descended into), and route packs through migrate_pack/verify_pack.

Directory packs are REPORTED as `dir-form-unsupported`, not rewritten in place:
a single-file pack is replaced atomically (a fully-built temp archive swapped in
with one os.replace), but a populated directory can't be swapped that way, so an
interrupted in-place rewrite could leave an authoring pack half-migrated. The
status is a problem status, so it counts against the run's exit code and shows
in the summary — the operator re-packs or migrates it as a `.feedpak` instead of
it vanishing from the report. Addresses a CodeRabbit review finding.

Signed-off-by: Kris Anderson <topkoa@gmail.com>

* fix(migrate): verify requires an explicit `off` on a retained full mix

verify_zip accepted any non-truthy `default` on a multi-stem `full` (missing,
empty, boolean, `false`/`no`/`0`, malformed) as "ok". But core defaults an ABSENT
`default` to True — ON (lib/sloppak.py: `s.get("default", True)`) — and treats an
empty/unrecognized string as ON too, so a migrated-shape pack whose `full` stem
has a missing or blank default beside instrument stems would actually play the
mixdown on open and double the song. verify was certifying that as safe.

Require an explicit normalized `off` beside instrument stems: `on`-ish values are
reported `full-stem-default-on` (actively plays), everything that is not a
normalized `off` is reported `full-stem-default-not-off`. The migrator already
writes the literal `off`, so its own output is unaffected; this also certifies
the pack is in the tool's canonical, most-portable shape. The len>1 gate is kept,
so a sole `full` stem (which IS the audio) is not policed.

Adds parametrized coverage for missing / empty / boolean / off-ish / malformed
defaults, and a sole-full-stem case. Addresses a CodeRabbit review finding.

Signed-off-by: Kris Anderson <topkoa@gmail.com>

---------

Signed-off-by: Kris Anderson <topkoa@gmail.com>
Co-authored-by: Kris Anderson <topkoa@gmail.com>

CHANGELOG.md

@@ -7,6 +7,40 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+- **The full mix is a stem** (#933) — core no longer depends on `original_audio:`, a
+  top-level manifest key this repo invented (#583) that the feedpak spec never had.
+  The format already carried the pre-separation mixdown as a stem; feedpak 1.15.0
+  (feedpak-spec#53) reserves the id **`full`** for it, so that is where core reads it
+  from now.
+
+  `full` is a mixdown, not a layer — it already contains every instrument — so
+  `load_song()` lifts it OUT of `LoadedSloppak.stems` onto `LoadedSloppak.full_mix`.
+  Nothing that sums stems or renders one fader per stem can see it, which is what
+  makes retaining it safe; leaving it in the list would double the whole song and
+  leave "guitar" audible with the guitar fader muted. That trap is exactly why the
+  packer invented the key instead of putting the mixdown where the format says it
+  goes — the bug was in the reader, and this fixes the reader.
+
+  Consequences worth knowing:
+  - The highway WS `song_info` frame gains `full_mix_url` / `has_full_mix`.
+    `original_audio_url` / `has_original_audio` remain as **deprecated aliases**
+    (same values) for one release so a client built against the old frame keeps
+    working; they go with the fallback below (#945).
+  - `stems` on `song_info`, and `stem_ids` / `stem_count` in the library index, now
+    describe *instrument* stems only — a separated pack that retains its mixdown no
+    longer advertises a bogus "full" stem chip or an inflated stem count.
+  - Audio fingerprinting (`lib/enrichment.py`) now resolves the mixdown the same
+    way, which **widens** its coverage: it previously returned `None` for any pack
+    without the invented key, so fingerprinting silently did nothing for the
+    overwhelming majority of packs.
+  - Core still **reads** `original_audio:` as a deprecated fallback, because every
+    pack written before the spec caught up carries it and would otherwise lose its
+    pristine mix. `tools/migrate_full_mix_stem.py` rewrites those packs into the
+    spec shape (moves `original/full.ogg` → `stems/full.ogg`, adds the `full` stem
+    at `default: off`, drops the key); the fallback and the aliases are removed once
+    they are migrated (#945).
+
 ### Added
 - **Genres fall back to MusicBrainz enrichment** — the effective genre now
   resolves override → pack genre → the enrichment match's primary genre