refactor(sync): extract stats-sync engine behind a DB-driver interface

Move snapshot/merge/quiescence engine and ssh helpers from launcher/sync
into src/core/services/stats-sync, parameterized on a minimal SyncDb
driver. The launcher binds it to bun:sqlite via launcher/sync/bun-driver;
the sync command becomes a thin adapter over the shared sync flow.

launcher/sync/sync-db.ts

@@ -1,105 +1,12 @@
-import fs from 'node:fs';
-import { Database } from 'bun:sqlite';
-import {
-  LexiconResolver,
-  mergeAnime,
-  mergeExcludedWords,
-  mergeMediaMetadata,
-  mergeVideos,
-} from './merge-catalog.js';
-import { mergeSessions } from './merge-sessions.js';
-import { copyRemoteOnlyRollups, refreshRollupsForNewSessions } from './merge-rollups.js';
-import {
-  assertMergeableSchema,
-  createEmptyMergeSummary,
-  type SyncMergeSummary,
-} from './sync-shared.js';
+import { mergeSnapshotIntoDb as mergeSnapshotIntoDbWith } from '../../src/core/services/stats-sync/merge.js';
+import { openBunSyncDb } from './bun-driver.js';
+import type { SyncMergeSummary } from './sync-shared.js';
 
 export type { SyncMergeSummary } from './sync-shared.js';
 export { createDbSnapshot, findLiveStatsDaemonPid } from './sync-shared.js';
+export { formatMergeSummary } from '../../src/core/services/stats-sync/merge.js';
 
-/**
- * Merge a snapshot of another machine's immersion database into the local
- * one. Insert-only union keyed on natural keys (session_uuid, video_key,
- * normalized_title_key, word/kanji identity); lifetime and rollup aggregates
- * are updated incrementally so history older than the session retention
- * window is preserved on both sides. Idempotent: re-merging the same
- * snapshot is a no-op.
- */
+/** bun:sqlite binding of the shared snapshot-merge engine. */
 export function mergeSnapshotIntoDb(localDbPath: string, snapshotPath: string): SyncMergeSummary {
-  if (!fs.existsSync(localDbPath)) {
-    throw new Error(`Local stats database not found: ${localDbPath}`);
-  }
-  if (!fs.existsSync(snapshotPath)) {
-    throw new Error(`Snapshot database not found: ${snapshotPath}`);
-  }
-
-  const remote = new Database(snapshotPath, { readonly: true });
-  let local: Database;
-  try {
-    local = new Database(localDbPath, { readwrite: true, create: false });
-  } catch (error) {
-    remote.close();
-    throw error;
-  }
-  try {
-    assertMergeableSchema(remote, 'Snapshot');
-    assertMergeableSchema(local, 'Local');
-
-    const summary = createEmptyMergeSummary();
-    local.run('PRAGMA foreign_keys = ON');
-    local.run('PRAGMA busy_timeout = 5000');
-    local.run('BEGIN IMMEDIATE');
-    try {
-      const animeIdMap = mergeAnime(local, remote, summary);
-      const { videoIdMap, addedVideoIds } = mergeVideos(local, remote, animeIdMap, summary);
-      mergeMediaMetadata(local, remote, videoIdMap, addedVideoIds);
-      mergeExcludedWords(local, remote, summary);
-
-      const lexicon = new LexiconResolver(local, remote, summary);
-      const { newSessionIds } = mergeSessions(
-        local,
-        remote,
-        videoIdMap,
-        animeIdMap,
-        lexicon,
-        summary,
-      );
-      lexicon.applyFrequencyDeltas();
-      refreshRollupsForNewSessions(local, newSessionIds, summary);
-      copyRemoteOnlyRollups(local, remote, videoIdMap, summary);
-
-      local.run('COMMIT');
-      return summary;
-    } catch (error) {
-      local.run('ROLLBACK');
-      throw error;
-    }
-  } finally {
-    local.close();
-    remote.close();
-  }
-}
-
-export function formatMergeSummary(summary: SyncMergeSummary): string {
-  const lines = [
-    `Sessions merged: ${summary.sessionsMerged} (${summary.sessionsAlreadyPresent} already present, ${summary.activeSessionsSkipped} unfinished skipped)`,
-  ];
-  const detail: string[] = [];
-  if (summary.animeAdded) detail.push(`${summary.animeAdded} series`);
-  if (summary.videosAdded) detail.push(`${summary.videosAdded} videos`);
-  if (summary.wordsAdded) detail.push(`${summary.wordsAdded} words`);
-  if (summary.kanjiAdded) detail.push(`${summary.kanjiAdded} kanji`);
-  if (summary.subtitleLinesAdded) detail.push(`${summary.subtitleLinesAdded} subtitle lines`);
-  if (summary.excludedWordsAdded) detail.push(`${summary.excludedWordsAdded} excluded words`);
-  if (detail.length > 0) lines.push(`Added: ${detail.join(', ')}`);
-  if (summary.dailyRollupsCopied || summary.monthlyRollupsCopied) {
-    lines.push(
-      `Historical rollups copied: ${summary.dailyRollupsCopied} daily, ${summary.monthlyRollupsCopied} monthly`,
-    );
-  }
-  if (summary.rollupGroupsRecomputed) {
-    lines.push(`Rollup groups recomputed: ${summary.rollupGroupsRecomputed}`);
-  }
-  return lines.join('\n');
+  return mergeSnapshotIntoDbWith(openBunSyncDb, localDbPath, snapshotPath);
 }