plan/spec: pipeline-style updates are done

M3's row and §6 updated: the operator corpus is 125 cases across seven files,
all green, and the pipeline work also gave `aggregate` the `$replaceWith`
stage it never had -- the compiler is shared between the two.

Full matrix at this commit: 249/249 unit tests in ReleaseFast and ReleaseSafe,
83/83 fuzz, operators 125/0, positional 51/0, aggregation 70/0, pinned crud
228/63/196, e2e and crash-fuzz green.

PLAN.md

@@ -467,7 +467,7 @@ answers, and the trade is only acceptable because the lie is removed first.
 | M1 | **Cursors + wire polish** | getMore / killCursors / batchSize; server-side cursor state with idle timeout; sessions plumbing (lsid accepted) as drivers send it; hello advertisement updates; **`moreToCome` on requests** (see the bug below); command-monitoring assertions in the spec runner | crud spec suite green; e2e green |
 | M2 | **The `aggregate` command surface** | `$out` and `$merge` (7 of the 13 failures), and refusing every pipeline construct the engine does not implement instead of answering `0` (amendment A6). The other 6 failures are blocked on M2.5, M4 and M8 — see `docs/M2_DESIGN_REVIEW.md` §7 | `aggregate-*.json`: 0 fail among the 7 reachable cases |
 | M2.5 | **The aggregation engine** | expression evaluator, per-stage document iterator, the accumulators, `$unwind`; `$lookup`/`$facet` explicitly out of the first cut (amendment A6) | a purpose-built stage corpus, every expectation measured against mongod |
-| M3 | **Update operators + index types** | `distinct` (**done**); positional paths refused rather than destructive (**done**); `$`/`$[]`/`$[<ident>]` + `arrayFilters` (**done**); $setOnInsert, $addToSet, $mul, $min/$max, $pop, $pullAll, $currentDate + `$push`'s modifiers (**done**); then pipeline updates; partial + hashed indexes | `tests/spec/positional/` 0 fail (51 cases) and `tests/spec/operators/` 0 fail (102 cases), both recorded from mongod — **green**; the named gate could not see either, see below; remaining crud coverage; e2e3/e2e4 green |
+| M3 | **Update operators + index types** | `distinct` (**done**); positional paths refused rather than destructive (**done**); `$`/`$[]`/`$[<ident>]` + `arrayFilters` (**done**); $setOnInsert, $addToSet, $mul, $min/$max, $pop, $pullAll, $currentDate + `$push`'s modifiers (**done**); pipeline-style updates (**done**); then partial + hashed indexes | `tests/spec/positional/` 0 fail (51 cases) and `tests/spec/operators/` 0 fail (125 cases), both recorded from mongod — **green**; the named gate could not see either, see below; remaining crud coverage; e2e3/e2e4 green |
 | M4 | **Sessions + transactions** | logical sessions, snapshot isolation on the mmap engine, write concern at commit | sessions + transactions spec suites green |
 | M5 | **Change streams** | change feed + resume tokens (likely log-seq based), getMore integration | change-streams spec suite green |
 | M6 | **Admin/ops commands** | dbStats, collStats, serverStatus, ping, buildInfo, listDatabases filters, dropDatabase durability (log it) | mongosh UX smoke; e2e green |
@@ -1139,6 +1139,15 @@ has to be its own commit with its own re-recorded scorecard.
   in `build_upsert_doc`; the pinned crud corpus never noticed because its
   upsert cases match `upsertedId` loosely.
 
+  Pipeline-style updates went into the same corpus rather than an eighth
+  directory -- another shape of update document, sharing the recorder and the
+  masking. 23 cases, and the one that decides the design: **the `_id` survives
+  every stage**, including `$replaceRoot` and `$project: {_id: 0}`, because a
+  pipeline update rewrites a document rather than replacing one document with
+  another. A stage setting it to something else is ImmutableField (66). That
+  work also gave `aggregate` `$replaceWith`, which it never had, since the
+  stage compiler is shared.
+
   Left open, measured and deliberate: `$bit` is not implemented (mongod has it,
   no driver in `tests/` sends it, and it is not in the M3 row); `$sort` inside
   `$push` accepts only a one-field key document, where mongod allows several;

tests/spec/operators/README.md

@@ -71,8 +71,8 @@ push-modifiers.json    21 pass   0 fail   0 skip
 set-on-insert.json      9 pass   0 fail   0 skip
 ```
 
-It was recorded red — 18 pass / 84 fail against a server with none of these
+It was recorded red — 18 pass / 84 fail for the operators, 0 / 23 for the
-operators — and driven green by seven commits. The 18 that passed then were
+pipeline file — and driven green by eight commits. The 18 that passed then were
 the shapes this server already answered mongod's way, mostly refusals that
 happened to agree.