fix(subtitles): suppress per-glyph typesetting walls in live subtitle text

When embedded-track extraction is skipped (network-mounted media), live mpv
text during per-glyph typeset karaoke is a wall of simultaneous one-glyph
lines plus the syllable being typed. No parsed cues exist to substitute, so
the wall reached both overlays and recording verbatim. Detect bursts of many
single-glyph lines in the live fallback paths and drop them with their short
syllable companions, keeping concurrent dialogue lines.

src/core/services/ass-text.test.ts

@@ -10,6 +10,7 @@ import {
   isAssTemporalCommand,
   normalizePlainSubtitleText,
   parseAssEffectField,
+  removeLiveGlyphFragmentLines,
   removeAssControlDebrisLines,
 } from './ass-text';
 
@@ -202,3 +203,18 @@ test('isAnimatedAssEffectKind covers the stock animated effects only', () => {
   assert.equal(isAnimatedAssEffectKind('other'), false);
   assert.equal(isAnimatedAssEffectKind('none'), false);
 });
+
+test('removeLiveGlyphFragmentLines drops a per-glyph typesetting wall and its syllable', () => {
+  const wall = [...'wansdumretoikhI'].join('\n');
+  assert.equal(removeLiveGlyphFragmentLines(`${wall}\ntai`), '');
+});
+
+test('removeLiveGlyphFragmentLines keeps concurrent dialogue beside a glyph wall', () => {
+  const wall = [...'wansdumretoikhI'].join('\n');
+  assert.equal(removeLiveGlyphFragmentLines(`${wall}\nそれよりも　ノート…`), 'それよりも　ノート…');
+});
+
+test('removeLiveGlyphFragmentLines leaves ordinary short lines alone', () => {
+  const text = 'え\nはい。\nそうだな';
+  assert.equal(removeLiveGlyphFragmentLines(text), text);
+});

src/core/services/ass-text.ts

@@ -108,6 +108,24 @@ export function removeAssControlDebrisLines(text: string): string {
     .join('\n');
 }
 
+const MIN_GLYPH_BURST_LINES = 6;
+const MAX_GLYPH_BURST_COMPANION_GLYPHS = 3;
+
+/**
+ * Per-glyph karaoke typesetting flattened into live text becomes a wall of
+ * single-character lines plus the short syllable currently being typed. No authored
+ * subtitle stacks this many one-glyph lines at once, so when the wall is present drop
+ * it and its short companion fragments while keeping any concurrent dialogue line.
+ */
+export function removeLiveGlyphFragmentLines(text: string): string {
+  const lines = text.split('\n');
+  const singleGlyphLines = lines.filter((line) => [...line.trim()].length === 1).length;
+  if (singleGlyphLines < MIN_GLYPH_BURST_LINES) return text;
+  return lines
+    .filter((line) => [...line.trim()].length > MAX_GLYPH_BURST_COMPANION_GLYPHS)
+    .join('\n');
+}
+
 export interface NormalizePlainSubtitleTextOptions {
   /** Fold every line break into a single space. */
   collapseLineBreaks?: boolean;