api: document learned-topic drill rule

api/README.md

@@ -27,6 +27,11 @@ A problem's first-ever log enters at `+2` regardless of result. Stage names
 the NEXT review's interval. All dates are ET calendar strings anchored at
 noon UTC (`src/srs.ts`) — never raw `Date` math.
 
+Blind drills draw only from topics **already learned**: scheduled in an
+earlier week (the current week's optional pool is reserved for Saturday's
+gate) and with at least one core problem on the ladder — a skipped learning
+day never feeds drills just because its calendar week lapsed.
+
 ## Routes
 
 | Route | Auth | Purpose |