docs: the fix for original_audio is removal, not adoption

The spec already carries the pre-separation mixdown as a stem
({id: full, file: stems/full.ogg}), so the key added a second, redundant
location for audio to a format that already had one. Adopting it into the
spec would make that permanent; the resolution in #933 is to remove it.

No behaviour change — the gate is agnostic about which way a violation
resolves, and only insists that one of the two happens deliberately and in
the open before the code merges. This just stops the exception entry, the
changelog, and the docs from presupposing adoption.

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

CHANGELOG.md

@@ -21,7 +21,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   A key that must ship ahead of the spec uses the reserved `x-` prefix (always allowed) or is recorded in
   `feedpak-spec-exceptions.yml` with a tracking issue — and the gate fails if such an exception goes stale,
   so the allowlist can't become somewhere drift hides. `original_audio` is seeded there against #933 so the
-  gate lands green and starts blocking the *next* instance immediately. Docs: [docs/feedpak-spec-gate.md](docs/feedpak-spec-gate.md).
+  gate lands green and starts blocking the *next* instance immediately; the gate takes no position on how
+  #933 resolves (the expected outcome is removing the key, since the spec already carries the mixdown as a
+  stem — not adopting it). Docs: [docs/feedpak-spec-gate.md](docs/feedpak-spec-gate.md).
 
 ### Removed
 - **The classic v2 UI shell is gone — v3 is the only UI (R3a).** `static/index.html`, the