docs: library comments say what the scanner and resolver do, not how they came to be (#6277)

Rewrites the internal/library comment blocks to rule 32. Task, milestone,
decision and rule ids are gone; the incidents behind them are restated as
the constraint they left (why a file is deleted before its row, why the
missing mark is a mark, why duplicates block on duration). Counts and
album names from one library are dropped from production comments; test
comments keep the names their fixtures use. The tagReadVersion log keeps
each version's before and after, without the measurements.

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

internal/library/fingerprint_backfill_test.go

@@ -151,11 +151,11 @@ func TestFingerprintBackfill_Integration(t *testing.T) {
 		t.Errorf("coverage buckets %+v do not sum to the total", cov)
 	}
 
-	// 5. Changing the length (#3913) makes every stored row stale at once — the
+	// 5. Changing the length makes every stored row stale at once — the
 	// gauge shows the whole library pending before the backfill has touched a
 	// file — and the next pass re-derives each at the new length. The failure
-	// this prevents is invisible from the UI: prints at two lengths that silently
-	// never match.
+	// this prevents is invisible from the UI: prints at two lengths that
+	// silently never match.
 	shorter := DefaultFingerprintSettings
 	shorter.ChromaprintLengthSec = 60
 	if _, err := settings.Set(ctx, shorter); err != nil {