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-fixture.ts

@@ -18,6 +18,9 @@ export function createImmersionDbFixture(dbPath: string): void {
     db.prepare(
       `INSERT INTO imm_rollup_state(state_key, state_value) VALUES ('last_rollup_sample_ms', 0)`,
     ).run();
+    db.prepare(
+      `INSERT INTO imm_rollup_state(state_key, state_value) VALUES ('lexical_daily_rollups_version', 2)`,
+    ).run();
     db.prepare(
       `INSERT INTO imm_lifetime_global(global_id, CREATED_DATE, LAST_UPDATE_DATE) VALUES (1, ?, ?)`,
     ).run(String(Date.now()), String(Date.now()));

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(