PLAN §3 gives M2 one line of scope and one line of gate, and §6 lists the three
questions it deferred. This answers them, and reports one finding that has to
be settled before the rest is worth discussing: the gate named in the plan does
not exist. `mongodb/specifications` has no aggregation suite -- the thirteen
`aggregate-*.json` files we run live inside `crud` and test the aggregate
*command* surface, not stages. $lookup, $unwind, $facet, $addFields and
$replaceRoot appear nowhere in the pinned corpus.
Measured, not recalled. Seven stages exist, not the nine an earlier note in
this project claimed -- $set and $unset are update operators. There is no
expression engine: $field paths are open-coded in two places inside run_group
and $sum is the only accumulator. Probed on a live server, six of eight
pipelines answer ok:1 with a wrong result -- $avg, $max and $push all return
0, a compound _id groups everything into one bucket keyed by the unevaluated
expression, $literal is echoed back, and a $match after a $project still sees
the projected-away field. Only $addFields fails honestly.
That drives the tiering. M2 is not "add stages to the chain": the stream is a
materialized window and each stage moves its bounds, which cannot express
1->N ($unwind), a second collection ($lookup), or sub-pipelines ($facet), and
$project is not a stage transform at all. Six tiers proposed, four gate options
priced, and a recommendation: Tier 0 (refuse what is not implemented) alone
first, for the same reason expectEvents preceded the free list in M1 -- it will
move the scorecard down, and that is the point.
No decision is recorded in PLAN.md yet; that is what the review is for.
