The design review found the milestone's gate names a corpus that is not there:
`mongodb/specifications` has no aggregation suite, and the thirteen
`aggregate-*.json` files this project runs live inside `crud` and test the
aggregate *command*, not stages. One name was covering two milestones.
M2 becomes the command surface -- $out, $merge, db.aggregate(), collation,
let -- gated on the corpus that already exists, at 0 fail. M2.5 becomes the
engine: expression evaluator, per-stage document iterator, accumulators,
$unwind; $lookup and $facet explicitly out of the first cut, gated on a
corpus this project writes with every expectation measured against mongod.
Command surface first, engine second, chosen with the cost stated: the engine
is what gives wrong answers now -- six of eight probed pipelines answer ok:1
with a wrong result -- so this order leaves them alive a milestone longer.
Which is why M2 carries the refusals: every unimplemented construct stops
answering 0 and starts answering an error with a measured code, the same move
expectEvents made before the free list in M1. It will push the scorecard down,
and that is the point.
The review keeps its pre-decision recommendation verbatim, so the argument the
decision overrode stays legible.
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