fix(mpv): stabilize hover token subtitle highlighting

# Conflicts:
#	src/core/services/ipc.ts
#	src/main.ts

src/main/runtime/mpv-hover-highlight.test.ts

@@ -0,0 +1,147 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+
+import { PartOfSpeech, type SubtitleData } from '../../types';
+import {
+  HOVER_TOKEN_MESSAGE,
+  HOVER_SCRIPT_NAME,
+  buildHoveredTokenMessageCommand,
+  buildHoveredTokenPayload,
+  createApplyHoveredTokenOverlayHandler,
+} from './mpv-hover-highlight';
+
+const SUBTITLE: SubtitleData = {
+  text: '昨日は雨だった。',
+  tokens: [
+    {
+      surface: '昨日',
+      reading: 'きのう',
+      headword: '昨日',
+      startPos: 0,
+      endPos: 2,
+      partOfSpeech: PartOfSpeech.noun,
+      isMerged: false,
+      isKnown: false,
+      isNPlusOneTarget: false,
+    },
+    {
+      surface: 'は',
+      reading: 'は',
+      headword: 'は',
+      startPos: 2,
+      endPos: 3,
+      partOfSpeech: PartOfSpeech.particle,
+      isMerged: false,
+      isKnown: true,
+      isNPlusOneTarget: false,
+    },
+    {
+      surface: '雨',
+      reading: 'あめ',
+      headword: '雨',
+      startPos: 3,
+      endPos: 4,
+      partOfSpeech: PartOfSpeech.noun,
+      isMerged: false,
+      isKnown: false,
+      isNPlusOneTarget: true,
+    },
+    {
+      surface: 'だった。',
+      reading: 'だった。',
+      headword: 'だ',
+      startPos: 4,
+      endPos: 8,
+      partOfSpeech: PartOfSpeech.other,
+      isMerged: false,
+      isKnown: false,
+      isNPlusOneTarget: false,
+    },
+  ],
+};
+
+test('buildHoveredTokenPayload normalizes metadata and strips empty tokens', () => {
+  const payload = buildHoveredTokenPayload({
+    subtitle: SUBTITLE,
+    hoveredTokenIndex: 2,
+    revision: 5,
+  });
+
+  assert.equal(payload.revision, 5);
+  assert.equal(payload.subtitle, '昨日は雨だった。');
+  assert.equal(payload.hoveredTokenIndex, 2);
+  assert.equal(payload.tokens.length, 4);
+  assert.equal(payload.tokens[0]?.text, '昨日');
+  assert.equal(payload.tokens[0]?.index, 0);
+  assert.equal(payload.tokens[1]?.index, 1);
+  assert.equal(payload.colors.hover, 'E7C06A');
+});
+
+test('buildHoveredTokenMessageCommand sends script-message-to subminer payload', () => {
+  const payload = buildHoveredTokenPayload({
+    subtitle: SUBTITLE,
+    hoveredTokenIndex: 0,
+    revision: 1,
+  });
+
+  const command = buildHoveredTokenMessageCommand(payload);
+
+  assert.equal(command[0], 'script-message-to');
+  assert.equal(command[1], HOVER_SCRIPT_NAME);
+  assert.equal(command[2], HOVER_TOKEN_MESSAGE);
+
+  const raw = command[3] as string;
+  const parsed = JSON.parse(raw);
+  assert.equal(parsed.revision, 1);
+  assert.equal(parsed.hoveredTokenIndex, 0);
+  assert.equal(parsed.subtitle, '昨日は雨だった。');
+  assert.equal(parsed.tokens.length, 4);
+});
+
+test('createApplyHoveredTokenOverlayHandler sends clear payload when hovered token is missing', () => {
+  const commands: Array<(string | number)[]> = [];
+  const apply = createApplyHoveredTokenOverlayHandler({
+    getMpvClient: () => ({
+      connected: true,
+      send: ({ command }: { command: (string | number)[] }) => {
+        commands.push(command);
+        return true;
+      },
+    }),
+    getCurrentSubtitleData: () => SUBTITLE,
+    getHoveredTokenIndex: () => null,
+    getHoveredSubtitleRevision: () => 3,
+  });
+
+  apply();
+
+  const parsed = JSON.parse(commands[0]?.[3] as string);
+  assert.equal(parsed.hoveredTokenIndex, null);
+  assert.equal(parsed.subtitle, null);
+  assert.equal(parsed.tokens.length, 0);
+});
+
+test('createApplyHoveredTokenOverlayHandler sends highlight payload when hover is active', () => {
+  const commands: Array<(string | number)[]> = [];
+  const apply = createApplyHoveredTokenOverlayHandler({
+    getMpvClient: () => ({
+      connected: true,
+      send: ({ command }: { command: (string | number)[] }) => {
+        commands.push(command);
+        return true;
+      },
+    }),
+    getCurrentSubtitleData: () => SUBTITLE,
+    getHoveredTokenIndex: () => 0,
+    getHoveredSubtitleRevision: () => 3,
+  });
+
+  apply();
+
+  const parsed = JSON.parse(commands[0]?.[3] as string);
+  assert.equal(parsed.hoveredTokenIndex, 0);
+  assert.equal(parsed.subtitle, '昨日は雨だった。');
+  assert.equal(parsed.tokens.length, 4);
+  assert.equal(commands[0]?.[0], 'script-message-to');
+  assert.equal(commands[0]?.[1], HOVER_SCRIPT_NAME);
+});