fix(subtitles): preserve lyric selection and deduplicate secondary text

- Seek sidebar selections past overlapping karaoke exit spans
- Collapse duplicate long ASS lines and omit reconstructed sign grids

src/core/services/secondary-subtitle-line-identity.ts

@@ -0,0 +1,15 @@
+const MIN_FLATTENED_DUPLICATE_LENGTH = 16;
+const TERMINAL_SENTENCE_PUNCTUATION = /[.!?。！？…⋯]+$/gu;
+
+/**
+ * Identifies long lines that become duplicates when positioned ASS events are
+ * flattened into the secondary subtitle bar. Short dialogue stays distinct.
+ */
+export function flattenedSecondarySubtitleLineIdentity(text: string): string | null {
+  const identity = text
+    .normalize('NFKC')
+    .replace(/\s+/gu, '')
+    .replace(TERMINAL_SENTENCE_PUNCTUATION, '');
+
+  return identity.length >= MIN_FLATTENED_DUPLICATE_LENGTH ? identity : null;
+}