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/README.md

@@ -97,3 +97,19 @@ on :27019, runs the same driver workload against each (durable writes:
 mongo-lite fsyncs per command, mongod runs with `j: true`), measures kill -9
 reopen for both, and prints a side-by-side table. `compare.js` alone runs
 one side (see its `--help`-style header comment).
+
+### Iteration-to-iteration comparison: `bench-run.sh` + `concurrent.js`
+
+```sh
+bash tests/e2e/bench-run.sh [size] [doc-size] ["clients..."]   # e.g. 1g 16k "1 4 8 16 32"
+```
+
+Runs the main suite (`compare-run.sh`) plus a concurrent durable-write
+comparison (`concurrent.js`, N clients each doing sequential `insertOne`
+with `{w:1, j:true}` — the group-commit path under real contention), then
+writes a machine-readable, versioned report to
+`tests/e2e/results/bench-<timestamp>.txt` and prints a diff of the
+mongo-lite numbers against the previous run (`results/bench-latest.txt`).
+The report has `[main]` / `[concurrency]` / `[meta]` sections with
+`name<TAB>value` rows; `bench-run.sh 1g 16k` reproduces the phase8 gate
+(see `results/phase8.txt`).