fix(stats): version lexical rollups and preserve backfill writes

- Persist vocabulary_visible on imm_words and filter rollups/triggers by it, matching the totals' exclusion rules
- Normalize legacy second/millisecond timestamps when bucketing rollup rows into local days
- Replace the boolean rollup-ready flag with a version key so schema changes trigger an atomic rebuild
- Preserve queued playback writes during rollup backfill instead of dropping them under queue pressure
- Time out the lexical rollup worker instead of hanging forever on no response
- Key concurrent vocabulary summary requests by their known-words snapshot so they no longer share stale state
- Bound stats dashboard retries for unfinished vocabulary loads before showing the inline Retry control

launcher/test-support/immersion-db-schema.ts

@@ -154,6 +154,7 @@ export const IMMERSION_DB_FIXTURE_DDL = `
     last_seen REAL,
     frequency INTEGER,
     frequency_rank INTEGER,
+    vocabulary_visible INTEGER NOT NULL DEFAULT 1 CHECK(vocabulary_visible IN (0, 1)),
     UNIQUE(headword, word, reading)
   );
   CREATE TABLE imm_kanji(