tests/spec: an aggregation corpus, recorded from mongod

M2.5's gate, built before the milestone it gates -- the same order that put
`expectEvents` before the free list in M1 and Tier 0 before everything in M2.

`mongodb/specifications` has no aggregation suite, which is amendment A6's
central finding, so this milestone has to bring its own. The hazard in a corpus
we author is obvious and fatal: it can encode our own bugs as expectations and
then agree with us forever. So the split is enforced by the tooling.
`sources/*.json` holds documents and pipelines and nothing else; `record.js`
asks a real mongod 8.3.7 what each pipeline answers and writes the unified-
format file from the reply. Inputs authored, expectations measured -- the
discipline that corrected three assumptions in M1's session work and every
error code in M2, where the alternative would have shipped both times.

No second runner. `run.js --suite-dir` points the existing one somewhere else,
so the entity model, the matchers, the skip accounting and `expectEvents` come
for free; a second runner would drift from the first exactly where it mattered.
`--scorecard` is refused with `--suite-dir`, because `scorecard.txt` is the crud
corpus's record and the milestones are compared against it -- writing it from an
unrelated run would replace that record silently.

Errors record the code and not the message: message text is mongod's to change
between releases. Group pipelines end in a `$sort`, because group output order
is unspecified and a case depending on it would fail for the wrong reason on
either server.

The first source covers `$group`: nine accumulators including the edge cases
that decide an implementation -- `$avg` over a group whose values are not
numbers, `$min` of a field no document has, `$push` skipping a missing field,
`$first`/`$last` against input order, grouping on an array, a compound `_id`.

Where it starts, run against the M2 tip:

    group-accumulators.json    9 pass   10 fail   0 skip

The nine include the four refusals M2 added, which answer with mongod's own
codes -- so the corpus already confirms that half. The ten are the milestone.
The crud corpus is unchanged at 201/90/196.

tests/spec/aggregate/README.md

@@ -0,0 +1,69 @@
+# The aggregation corpus
+
+`mongodb/specifications` has no aggregation suite. The thirteen
+`aggregate-*.json` files this project runs come from `crud` and test the
+aggregate *command* — cursors, read concern, the write stages, collation,
+`let`. They touch stages barely and expressions not at all: `$lookup`,
+`$unwind`, `$facet`, `$addFields` and `$replaceRoot` appear nowhere in the
+pinned corpus. That is PLAN amendment A6, and this directory is its
+consequence: M2.5 has to bring its own gate.
+
+## The one rule
+
+**Inputs are authored here; expectations are measured against a real mongod.**
+
+A corpus we write is a corpus that can encode our own bugs as expectations, and
+it would then agree with us forever. So `sources/*.json` holds documents and
+pipelines and nothing else, and `record.js` asks mongod 8.3.7 what each pipeline
+answers. It is the same discipline that corrected three assumptions in M1's
+session work and every error code in M2 — the alternative, in both cases, would
+have shipped.
+
+## Running it
+
+```sh
+node tests/spec/run.js --suite-dir tests/spec/aggregate
+```
+
+The same runner as the crud corpus, pointed elsewhere. Sharing it is the point:
+the entity model, the matchers, the skip accounting and `expectEvents` come for
+free, and a second runner would drift from the first exactly where it mattered.
+
+`--scorecard` is refused with `--suite-dir`: `tests/spec/scorecard.txt` is the
+crud corpus's record and the milestones are compared against it.
+
+## Re-recording
+
+```sh
+mongod --port 27099 --dbpath /tmp/mongo-corpus &
+node tests/spec/aggregate/record.js --mongod-port 27099
+```
+
+Writes `<name>.json` for every `sources/<name>.json`. The generated files are
+committed: they *are* the corpus, and regenerating them is how a disagreement
+with mongod gets re-measured rather than argued about.
+
+Two things to know when adding cases:
+
+- **End a `$group` pipeline with a `$sort`.** Group output order is unspecified,
+  and a case that depended on it would fail for the wrong reason on either
+  server.
+- **Errors record the code, not the message.** Message text is mongod's to
+  change between releases; a corpus that pinned it would break for the wrong
+  reason.
+
+Leave out any case whose answer depends on a server newer than the 4.4 this
+server reports — recording it from mongod 8.x and judging it against a 4.4
+answer measures the version gap, not the engine.
+
+## Where it stands
+
+Recorded against mongod 8.3.7, run against the M2 tip:
+
+```
+group-accumulators.json    9 pass   10 fail   0 skip
+```
+
+The nine include the four refusals M2 added, which answer with mongod's own
+codes. The ten are M2.5's work: `$avg`, `$min`, `$max`, `$first`, `$last`,
+`$push`, `$addToSet`, `$count`, and a compound `_id`.