tests/spec: a recorded corpus for the positional operators

M3's gate as named -- "remaining crud coverage; e2e3/e2e4 green" -- cannot
see this work. e2e3 and e2e4 contain zero positional paths, and the pinned
crud corpus covers `$[<identifier>]` only: no `$[]` case, no bare `$` case
anywhere in it. Both stayed green through a bug that replaced an array with
`{"$[i]": {...}}` and answered ok: 1. So M3 brings its own corpus, built the
way M2.5's was: inputs authored in `sources/`, every expectation recorded
from mongod 8.3.7, run through the shared runner with `--suite-dir`.

51 cases across the three spellings. It stands at 15 pass / 36 fail against
the refusal, which is the intended shape -- `expressions.json` was recorded
at 1 pass / 26 fail before the evaluator and is green now. The 15 that pass
are refusals where this server's code already matches; of the 36, 31 are the
constructs answering "not implemented" and 5 are refusals whose code
differs, four of them arrayFilters validation this server cannot do because
it never parses the option.

Every case records its `outcome`, refusals included. That is deliberate and
it is the whole point of the file: a refusal that left the document mangled
is indistinguishable from a clean one in `expectError` alone, and a mangled
document is what this corpus exists to catch.

What recording it settled, none of it guessable, the first contradicting
what the design review assumed:

  - `y.$[i].c.$[i].d`, one identifier reused at two levels, is **accepted**
    -- not a duplicate-identifier error
  - `$[]` over an empty array is a no-op with modifiedCount 0
  - `$[]` over an array with a non-document element is error 28, where every
    other path failure here is 2
  - a positional segment never creates: a missing or non-array path is an
    error, where `$set: {'a.b': 1}` would construct one
  - an upsert gets no special case -- it fails for the same reason
  - `$` writes only the first matching element, and is refused when the query
    never touched the array
  - any of the three in first position is refused, as is `$` twice in a path
  - `arrayFilters` alongside a replacement is ignored rather than refused

A case needing its own documents reseeds through operations rather than
`initialData`, because the format's `initialData` is per file and the runner
seeds it once per test. That keeps one file per operator instead of one file
per document shape.

crud scorecard unchanged at 204/87, aggregation corpus still 70/0.

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**); then $setOnInsert, $addToSet, $mul, $min/$max, $pop, $pullAll, $currentDate, `arrayFilters`, pipeline updates; partial + hashed indexes | remaining crud coverage; e2e3/e2e4 green |
+| M3 | **Update operators + index types** | `distinct` (**done**); positional paths refused rather than destructive (**done**); then `$`/`$[]`/`$[<ident>]` implemented, $setOnInsert, $addToSet, $mul, $min/$max, $pop, $pullAll, $currentDate, pipeline updates; partial + hashed indexes | `tests/spec/positional/` 0 fail (51 cases, recorded from mongod — the named gate could not see this work, see below); remaining crud coverage; e2e3/e2e4 green |
 | M4 | **Sessions + transactions** | logical sessions, snapshot isolation on the mmap engine, write concern at commit | sessions + transactions spec suites green |
 | M5 | **Change streams** | change feed + resume tokens (likely log-seq based), getMore integration | change-streams spec suite green |
 | M6 | **Admin/ops commands** | dbStats, collStats, serverStatus, ping, buildInfo, listDatabases filters, dropDatabase durability (log it) | mongosh UX smoke; e2e green |
@@ -1082,6 +1082,24 @@ has to be its own commit with its own re-recorded scorecard.
   is not excluded by the reasoning above; it holds no collection lock, so it
   is not this crash, but the two drops disagree about which lock protects the
   namespace and that wants one answer.
+- **M3's gate is `tests/spec/positional/`, not the one named in §3.** The row
+  said "remaining crud coverage; e2e3/e2e4 green". `e2e3` and `e2e4` contain
+  zero positional paths, and the pinned crud corpus covers `$[<identifier>]`
+  only — no `$[]` case, no bare `$` case anywhere in it. Both would have been
+  green through the worst version of the array-destroying bug. So M3 brings
+  its own corpus, built exactly like M2.5's: 51 cases, inputs authored in
+  `sources/`, every expectation recorded from mongod 8.3.7, run through the
+  shared runner with `--suite-dir`.
+
+  It stands at 15 pass / 36 fail against the refusal, which is the intended
+  shape — `expressions.json` was 1/26 before the evaluator and is green now.
+  Recording it settled a dozen things, and the first contradicts what the
+  design review assumed: `y.$[i].c.$[i].d` reusing one identifier at two
+  levels is **accepted**, not a duplicate-identifier error. Also: `$[]` over
+  an empty array is a no-op; `$[]` over an array with a non-document element
+  is error 28 where every other path failure is 2; a positional segment never
+  creates, so a missing or non-array path is an error where `$set: {'a.b': 1}`
+  would construct; and an upsert gets no special case.
 - **M3 update operators** — open. `distinct` landed first because it was a
   whole missing command with no dependencies, and measuring it turned up three
   things worth keeping, none of which are `distinct`'s to fix: