Use first >= 1d interval for starting memory state (#3959)

When a filtered revlog history begins with one or more < 1d intervals
(for example, because it starts in the middle of a sequence of
relearning steps), use the first >= 1d interval to calculate the initial
memory state.

Bug report thread:

https://forums.ankiweb.net/t/fsrs-stability-when-first-non-ignored-revlog-has-a-relearning-interval/59894

rslib/src/scheduler/fsrs/params.rs

@@ -313,9 +313,12 @@ pub(crate) fn reviews_for_fsrs(
         if entry.review_kind == RevlogReviewKind::Filtered && entry.ease_factor == 0 {
             continue;
         }
+        // For incomplete review histories, initial memory state is based on the first
+        // user-graded review after the cutoff date with interval >= 1d.
         let within_cutoff = entry.id.0 > ignore_revlogs_before.0;
         let user_graded = matches!(entry.button_chosen, 1..=4);
-        if user_graded && within_cutoff {
+        let interday = entry.interval >= 1 || entry.interval <= -86400;
+        if user_graded && within_cutoff && interday {
             first_user_grade_idx = Some(index);
         }
 
@@ -477,6 +480,7 @@ pub(crate) mod tests {
             review_kind,
             id: days_ago_ms(days_ago).into(),
             button_chosen: 3,
+            interval: 1,
             ..Default::default()
         }
     }
@@ -711,6 +715,28 @@ pub(crate) mod tests {
         assert_eq!(convert_ignore_before(revlogs, true, days_ago_ms(9)), None);
     }
 
+    #[test]
+    fn skip_initial_relearning_steps() {
+        let revlogs = &[
+            revlog(RevlogReviewKind::Review, 10),
+            RevlogEntry {
+                button_chosen: 1, // Again
+                interval: -600,
+                ..revlog(RevlogReviewKind::Review, 8)
+            },
+            revlog(RevlogReviewKind::Relearning, 8),
+            revlog(RevlogReviewKind::Review, 6),
+        ];
+        // | = Ignore before
+        // A = Again
+        // X = Relearning
+        // R | A X R
+        assert_eq!(
+            convert_ignore_before(revlogs, false, days_ago_ms(9)),
+            fsrs_items!([review(0)], [review(0), review(2)])
+        );
+    }
+
     #[test]
     fn ignore_before_date_between_learning_steps_when_reviewing() {
         let revlogs = &[