docs: the history phrases the id sweep missed, restated as present-tense facts or traps (#6277)

A wider pass for "previous", "before the fix", "we shipped" and the
like. The chi double-subtree, uppercase Vorbis key and stopPropagation
comments now describe the trap they guard; commit hashes, CI run
numbers and a deleted file name are gone; uuidString's comment says
what it returns.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/mbids_test.go

@@ -97,10 +97,9 @@ func TestExtractMBIDs_MissingTagsReturnEmpty(t *testing.T) {
 }
 
 func TestExtractMBIDs_VorbisUppercaseKeysMiss(t *testing.T) {
-	// Regression guard: real dhowden never produces uppercase Vorbis
-	// keys. If a future change starts looking up "MUSICBRAINZ_ALBUMID"
-	// directly, this test must keep returning empty so we don't
-	// silently re-introduce the v0 bug where every album skipped.
+	// Real dhowden never produces uppercase Vorbis keys. Looking up
+	// "MUSICBRAINZ_ALBUMID" directly would find nothing, and every album
+	// would be skipped; this test keeps that lookup returning empty.
 	m := stubMeta{
 		format: tag.VORBIS,
 		raw: map[string]interface{}{