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-latest.txt

@@ -0,0 +1,36 @@
+# mongo-lite vs MongoDB benchmark
+# date: 2026-08-03T08:53:32Z  git: c8d547f+dirty
+# args: size=1g doc-size=16k wc=j clients=1 4 8 16 32 per-client=2000
+# reproduce: bash tests/e2e/bench-run.sh 1g 16k "1 4 8 16 32"
+
+[main]
+insertOne (sequential) ×200	0.20 ms	14.8 ms
+bulk insert throughput	732.4 MB/s	546.2 MB/s
+docs loaded	65,536	65,536
+createIndex({k: 1})	74.4 ms	83.6 ms
+countDocuments({})	3.1 ms	12.4 ms
+findOne({_id: <ObjectId>})	0.59 ms	0.61 ms
+findOne({k: 500}) (indexed)	0.56 ms	0.94 ms
+find({p: {$gte,$lt}}).count() (scan)	13.0 ms	15.0 ms
+find({}).sort({_id:-1}).limit(20)	2.2 ms	2.2 ms
+find({}, {proj}).limit(1000)	3.5 ms	4.4 ms
+aggregate $group by k	8.3 ms	13.2 ms
+updateOne({_id}) ×50	0.14 ms	0.20 ms
+updateMany({k: 7}, {$inc})	1.9 ms	6.4 ms
+deleteOne({_id}) + insertOne	0.58 ms	4.9 ms
+node client RSS	158 MB	157 MB
+
+[concurrency]
+clients	1	8435	232	36.4x
+clients	4	22480	491	45.8x
+clients	8	25920	966	26.8x
+clients	16	31079	1842	16.9x
+clients	32	34041	3632	9.4x
+
+[meta]
+ml_rss_mb	553
+md_rss_mb	1486
+ml_reopen	0.8s
+md_reopen	1.3s
+ml_disk_mb	97MB
+md_disk_mb	106MB