plan: the index corpus is M3's last gate

42 cases at 3/39, and the two rows where recording it corrected the review:
`$in` in a partial filter is allowed, and a same-key different-filter clash
with no explicit name is IndexKeySpecsConflict (86) rather than 67.

Everything else at this commit is unmoved: 249/249 unit tests, operators
125/0, positional 51/0, aggregation 70/0, pinned crud 228/63/196.

PLAN.md

@@ -1143,6 +1143,14 @@ has to be its own commit with its own re-recorded scorecard.
   contrast, was honestly missing -- refused, with the wrong code (2 where
   mongod says 67) but the right answer.
 
+  **`tests/spec/indexes/` is the gate**, recorded red at 3/39 across 42 cases.
+  A case there is a *sequence* -- create, insert, read, list -- because an
+  index outlives a `deleteMany` and every case is about which indexes exist.
+  Recording it corrected two of the review's own guesses: `$in` in a partial
+  filter is **allowed** (the review grouped it with `$ne` and `$regex`, which
+  are 67), and the same key with a different filter and no explicit name is
+  **IndexKeySpecsConflict (86)**, not 67.
+
   `docs/M3_INDEX_TYPES_DESIGN_REVIEW.md` measures both features' rules and
   argues the planner rule that decides the design: a partial index may only
   answer a query whose predicates *imply* its filter, so until that test