tests/e2e: iteration-to-iteration benchmark harness

compare-run.sh answers "how do we compare to MongoDB"; it says nothing
about whether a change made things better or worse than last week. Add a
harness that records each run and diffs it against the previous one.

bench-run.sh wraps compare-run.sh, adds a concurrent durable-write
comparison (concurrent.js: N clients each doing sequential insertOne with
{w:1, j:true}, exercising the group-commit path under real contention),
writes a versioned name<TAB>value report to results/bench-<timestamp>.txt,
and prints a diff of our numbers against results/bench-latest.txt.

Also:
- compare-run.sh polled with fixed sleeps, which are flaky once earlier
  benchmark phases have warmed the machine; both servers now wait on a
  real driver connection instead.
- a dispatch error only reached the client as a generic InternalError,
  with nothing on the server side naming the failing command; log the
  connection, command and error name before replacing the reply.

tests/e2e/results/bench-20260803-112548.txt

@@ -0,0 +1,34 @@
+# mongo-lite vs MongoDB benchmark
+# date: 2026-08-03T08:26:46Z  git: c8d547f+dirty
+# args: size=128m doc-size=8k wc=j clients=1 2 4 per-client=2000
+# reproduce: bash tests/e2e/bench-run.sh 128m 8k "1 2 4"
+
+[main]
+insertOne (sequential) ×200	0.20 ms	5.9 ms
+bulk insert throughput	664.4 MB/s	357.9 MB/s
+docs loaded	16,384	16,384
+createIndex({k: 1})	10.4 ms	45.2 ms
+countDocuments({})	0.73 ms	8.6 ms
+findOne({_id: <ObjectId>})	0.35 ms	0.62 ms
+findOne({k: 500}) (indexed)	0.36 ms	1.2 ms
+find({p: {$gte,$lt}}).count() (scan)	2.4 ms	3.2 ms
+find({}).sort({_id:-1}).limit(20)	2.0 ms	2.2 ms
+find({}, {proj}).limit(1000)	—	4.6 ms
+aggregate $group by k	—	4.6 ms
+updateOne({_id}) ×50	—	0.26 ms
+updateMany({k: 7}, {$inc})	—	5.7 ms
+deleteOne({_id}) + insertOne	—	4.8 ms
+node client RSS	—	136 MB
+
+[concurrency]
+clients	1	8117	175	46.4x
+clients	2	17102	230	74.4x
+clients	4	21967	352	62.4x
+
+[meta]
+ml_rss_mb	158
+md_rss_mb	251
+ml_reopen	0.3s
+md_reopen	0.4s
+ml_disk_mb	7MB
+md_disk_mb	7MB