fix(stats): keep first_seen for words with retention-pruned history

- Only repair stale first_seen when frequency matches surviving occurrences
- Rows whose older lines were pruned (or predate occurrence tables) keep their original first_seen
- Extend rollup rebuild test with a retention-pruned word

src/core/services/immersion-tracker/lexical-rollups.test.ts

@@ -201,15 +201,17 @@ test('lexical rollup rebuild repairs seen bounds left pointing at deleted histor
     const staleLast = seen + 86_400.631;
     db.prepare(
       `INSERT INTO imm_words(id, headword, word, reading, first_seen, last_seen, frequency)
-       VALUES (1, '猫', '猫', 'ねこ', ?, ?, 1), (2, '犬', '犬', 'いぬ', ?, ?, 1)`,
+       VALUES (1, '猫', '猫', 'ねこ', ?, ?, 1), (2, '犬', '犬', 'いぬ', ?, ?, 1),
-    ).run(staleFirst, staleLast, seen - 0.1, seen - 0.1);
+         (3, '鳥', '鳥', 'とり', ?, ?, 5)`,
+      // Word 3's earlier history was pruned by retention: frequency still counts it.
+    ).run(staleFirst, staleLast, seen - 0.1, seen - 0.1, staleFirst, seen);
     db.prepare(
       `INSERT INTO imm_kanji(id, kanji, first_seen, last_seen, frequency) VALUES (1, '猫', ?, ?, 1)`,
     ).run(staleFirst, staleLast);
     db.prepare(
       `INSERT INTO imm_word_line_occurrences(line_id, word_id, occurrence_count, seen_ms)
-       VALUES (?, 1, 1, ?), (?, 2, 1, ?)`,
+       VALUES (?, 1, 1, ?), (?, 2, 1, ?), (?, 3, 1, ?)`,
-    ).run(lineId, BASE_MS + 251, lineId, BASE_MS + 251);
+    ).run(lineId, BASE_MS + 251, lineId, BASE_MS + 251, lineId, BASE_MS + 251);
     db.prepare(
       `INSERT INTO imm_kanji_line_occurrences(line_id, kanji_id, occurrence_count, seen_ms)
        VALUES (?, 1, 1, ?)`,
@@ -231,8 +233,9 @@ test('lexical rollup rebuild repairs seen bounds left pointing at deleted histor
       [
         [seen, seen],
         [seen - 0.1, seen - 0.1],
+        [staleFirst, seen],
       ],
-      'healthy rows within flush lag are untouched',
+      'flush-lag and retention-pruned rows keep their first_seen',
     );
     assert.deepEqual(bounds('imm_kanji'), [[seen, seen]]);
   } finally {

src/core/services/immersion-tracker/lexical-rollups.ts

@@ -35,6 +35,12 @@ export function localEpochDaySql(value: string): string {
 // stamped at queue time, before seen_ms is stamped at flush, so healthy rows
 // never exceed their newest occurrence and only trail their oldest by flush lag.
 // Rows with undated occurrences are skipped.
+//
+// first_seen can also legitimately predate every surviving occurrence: retention
+// pruning cascades away old lines, and early history predates the occurrence
+// tables, while both keep the lifetime frequency. Deletes subtract frequency
+// exactly, so only rows whose frequency matches their surviving occurrences have
+// their whole history on hand and a provably stale first_seen.
 function repairStaleSeenBounds(db: DatabaseSync): void {
   for (const [table, occurrenceTable, col] of [
     ['imm_words', 'imm_word_line_occurrences', 'word_id'],
@@ -48,7 +54,11 @@ function repairStaleSeenBounds(db: DatabaseSync): void {
     db.exec(`
       UPDATE ${table}
       SET first_seen = ${seenSec('MIN')}
-      WHERE first_seen < ${seenSec('MIN')} - ${FIRST_SEEN_FLUSH_LAG_SLACK_SEC} AND ${hasNoUndated};
+      WHERE first_seen < ${seenSec('MIN')} - ${FIRST_SEEN_FLUSH_LAG_SLACK_SEC}
+        AND ${hasNoUndated}
+        AND frequency = (
+          SELECT SUM(o.occurrence_count) FROM ${occurrenceTable} o WHERE o.${col} = ${table}.id
+        );
       UPDATE ${table}
       SET last_seen = ${seenSec('MAX')}
       WHERE last_seen > ${seenSec('MAX')} AND ${hasNoUndated};