27 cases, recorded before a line of the evaluator is written, which is the
whole point of having built the recorder first: the expectations come from
mongod 8.3.7 rather than from what the implementation is about to do.
The corpus reads 1 pass / 26 fail, and the one pass is the unknown-operator
refusal M2 already answers correctly. Expressions are exercised through
`$group` because `_id` and the accumulator arguments are the only expression
positions that exist until `$addFields` and `$project`'s computed fields land;
testing them anywhere else would be testing a stage that is not there.
Ten things the recording settled that guessing would have got wrong:
$add over a missing field or null null -- not an error, and not 0
$add over a string error 7157723
$divide by zero error 4848401
$mod of -5 by 4 -1, the dividend's sign
$lt of a number and a string true, canonical type order
$and over -5 truthy
$not of a missing field true
$switch, no branch and no default error 40069
two operators in one expression error 15983, *not* $group's 40238
$subtract with one operand error 16020
The six error codes are in `ErrorCode` already, so the evaluator's refusals and
its runtime failures have somewhere measured to land. The evaluator itself is
the next commit and is not started.
191/191 unit tests, crud corpus unchanged at 201/90/196.
