cursors: server-side cursors for find, aggregate and the listing commands

Every reply came back in a single batch with `cursor.id = 0`, `getMore` was a
stub answering an empty `nextBatch` on the literal namespace `test.$cmd`, and
nothing read `batchSize`. That caps the useful collection size at what fits in
one 48 MiB message, which is the opposite of the tens-of-GB target and the
reason M0 made whole-index scans stream: the streaming candidate generator
existed with no consumer that could suspend.

## What a cursor is allowed to remember

A cursor holds no lock between requests, so everything it saves has to survive
arbitrary concurrent mutation. Nothing here is a pointer, and the two things
that look like stable addresses are not: `reset_tree` re-creates node ids 0 and
1 as different nodes, and `rebuild_collection` moves every document. Three
sources, chosen by query shape, each with a different memory contract:

- **stream** -- an index-ordered walk resumed from a `(key, off)` anchor plus a
  `(leaf, slot)` hint. O(key) state, so this is what lets a cursor walk a
  collection larger than memory. Survives a rebuild, because a repack changes
  no key.
- **offsets** -- the matched slab offsets a narrowed plan already materialized,
  8 bytes each. Killed by a rebuild with `QueryPlanKilled`, because those
  offsets now name unrelated bytes.
- **buffered** -- canonical BSON copies, for a sort no index provides and for
  aggregate/listing output. Depends on nothing, which is what lets a listing
  hold a cursor over a `$cmd.*` namespace no collection backs.

`Collection.layout_epoch` and `Index.epoch` are the invalidation tokens, both
checked as error returns rather than assertions since a client reaches them by
keeping a cursor open across maintenance.

## Resume

`resume_forward`/`resume_reverse` are O(1) while the hint holds and fall back to
an exact-order band walk bounded by `resume_walk_max`. Without the hint, `seek`
lands at the *start* of an equal-key band, so `sort({status: 1})` over three
distinct values across 10M documents would cost ~5e10 comparisons to drain.

Two hazards found by draining a collection while writing to it, neither
predictable from reading the code:

- A deleted anchor must resume at its *band position*, or the rest of an
  equal-key band is silently dropped -- most of the collection on a
  low-cardinality index. Hence `band_index`.
- On a **unique** index a same-key entry can only be the anchor rewritten, so
  resuming at it returned updated documents twice. Observed as duplicate `_id`s
  while updating underneath a drain.

## Protocol

Measured against mongod 8.3.7 rather than recalled, which corrected three
assumptions: a bare `getMore` does *not* inherit the find's `batchSize` (4998 of
5000 documents come back), a namespace mismatch is `Unauthorized` (13) not
`CursorNotFound`, and `CursorInUse` is 143 not 12051.
`internalQueryFindCommandBatchSize` is 101, `cursorTimeoutMillis` 600000,
`clientCursorMonitorFrequencySecs` 4.

The rule everything follows is **never look ahead**: a batch that met its target
leaves the cursor open even when the source is in fact exhausted, so four
documents at `batchSize: 2` take three commands. `limit` acts as an EOF source,
which is what makes `batchSize == limit` close in one round trip. `skip` is
consumed once. `batchSize: 0` returns an empty batch with a live cursor.

Cursor ids are `(nonce << 20) | slot`, always positive. The nonce is not
decoration: without it a recycled slot serves one client another's documents.
Cursors are not connection-pinned, since the driver spec allows a `getMore` on
any connection to the same server; they end at exhaustion, `killCursors`, or the
idle sweep (a second monitor fiber, separate from the TTL one because the
cadences differ by an order of magnitude and a TTL failure must not stop
reclamation). The registry is fixed-capacity and evicts the least recently used
cursor, whose client sees the same 43 an idle timeout gives.

Fixed alongside, because cursors are what expose them:

- `listCollections` reported `"<db>."` with an *empty* collection part, which
  makes the driver throw client-side -- so it would have broken the moment its
  cursor stopped being id 0. Now `<db>.$cmd.listCollections`, as mongod uses.
- `count` ignored `skip` and `limit` entirely.
- `wire.end_message` now bounds a reply by the 48 MiB we advertise rather than
  by `maxInt(u32)`; a reply past what we told the client to expect is not a large
  reply, it is a desynchronized connection.
- Two `codeName` strings were wrong: 72 is `InvalidOptions` (MongoDB has no
  `InvalidArgument`), and 40324 reports as `Location40324`.

## Verification

Unit 160/160 in ReleaseFast and ReleaseSafe; `tests/e2e/e2e7.js` adds 86 cursor
checks across five phases (batching/lifecycle/errors, streaming across churn,
aggregate+listings+count, expiry+capacity, restart) and is self-contained
because cursor behaviour is only observable with non-default flags. No
regressions: e2e 49, e2e3 16, e2e4 17, e2e2 2, e2e6 72. Spec 168 pass / 124
fail, +5 against the previous scorecard.

Mutation-checked, per the repo's second ground rule: `hint_slot + 1`, the
`band_index` off-by-one, both epoch bumps, the id nonce, the at-least-one-
document rule, and `stream_shape` returning null each turn the intended test
red. One claim was withdrawn rather than kept -- swapping `std.mem.order` for
`cmp_prefix` in the band walk changes nothing observable, so the comment now
says so instead of asserting a check that does not hold.

tests/e2e/README.md

@@ -41,6 +41,19 @@ node tests/e2e/e2e6.js              # 73 checks, ~15 s, needs no running server
 E2E6_PORT=27300 node tests/e2e/e2e6.js   # different port if 27220 is taken
 ```
 
+`e2e7.js` is the cursor suite, self-contained for a different reason: cursor
+behaviour is only observable with non-default flags. It spawns three servers in
+turn -- default flags for batching/streaming/aggregate, then
+`--cursor-timeout-ms 800 --cursor-sweep-secs 1 --max-open-cursors 4` for idle
+expiry and registry capacity, then a restart on the same database to confirm a
+cursor does not survive one. Most of it uses raw `runCommand`, because the
+driver hides `cursor.id` and that is the thing under test:
+
+```sh
+node tests/e2e/e2e7.js              # 86 checks, needs no running server
+E2E7_PORT=27310 node tests/e2e/e2e7.js   # different port if 27230 is taken
+```
+
 Rebuild with `zig build` after any change under `src/` before restarting the
 server: `zig build test` compiles the test binary only and leaves
 `zig-out/bin/multiforadb` stale, so the suites keep running against the old