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>

feedpak-spec-exceptions.yml

@@ -25,6 +25,13 @@ exceptions:
       Added by #583 (the full mix played while every stem fader sits at unity,
       since demucs recombination is lossy). Core, lib/enrichment.py, and the
       stems plugin all depend on it, but it was never added to the spec — the
-      drift this gate exists to prevent. Seeded here so the gate lands green and
-      starts blocking the *next* instance immediately; remove once the spec
-      adopts the key.
+      drift this gate exists to prevent.
+
+      The expected resolution is REMOVAL, not adoption: the spec already carries
+      the mixdown as a stem ({id: full, file: stems/full.ogg}), so this key added
+      a second, redundant location for audio to a format that already had one.
+      See #933.
+
+      Seeded here so the gate lands green and starts blocking the *next* instance
+      immediately, rather than blocking on #933. The entry goes away when core
+      stops reading the key.