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.

src/server.zig

@@ -143,9 +143,10 @@ fn handle_connection_inner(io: std.Io, stream: std.Io.net.Stream, server: *Serve
 
         reply.reset();
 
-        commands.dispatch(&ctx, &msg, &reply) catch {
+        commands.dispatch(&ctx, &msg, &reply) catch |err| {
             // Discard any partial reply (the client would read the first
             // ok field, which may already say 1) and send a clean error.
+            std.debug.print("mongo-lite: dispatch error on conn {d} cmd {s}: {s}\n", .{ connection_id, msg.command_name(), @errorName(err) });
             reply.pairs.clearRetainingCapacity();
             reply.put_error(
                 @intFromEnum(commands.ErrorCode.internal_error),