feat(evnode-txsim): keep-alive conn pool + pprof endpoint

Two diagnostic improvements for the load generator:

1. http.Transport.MaxIdleConnsPerHost defaults to 2 in stdlib.
   With --concurrency=8 (or higher), 6+ goroutines per cycle had
   to open fresh TCP+TLS sockets per request because the pool
   couldn't hold their idle conns between requests. Bump
   MaxIdleConns / MaxIdleConnsPerHost / MaxConnsPerHost to
   2*concurrency so every active sender has a reusable keep-alive
   socket, eliminating handshake churn from the hot path.

2. Always-on net/http/pprof on 127.0.0.1:6060. evnode-txsim is a
   load tester, not a production daemon, so cost of always serving
   profiling is acceptable; the payoff is being able to grab CPU
   profiles under live load without re-deploying the binary —
   `ssh -L 6060:127.0.0.1:6060 root@loadgen \
     go tool pprof http://localhost:6060/debug/pprof/profile?seconds=30`.

A profile captured this way under c=8 traced the per-request hot
path: 25.5% in kernel write(2), 25% in net/http body marshaling.
That diagnostic surfaced that the c6in.2xlarge loadgen was the
binding constraint for the experiment at ~22 MB/s, not evnode or
DA — a finding we'd have spent another debug round chasing
without the in-process profiler.

Author: walldiss

tools/talis/cmd/evnode-txsim/main.go

@@ -26,6 +26,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	_ "net/http/pprof"
 	"os"
 	"os/signal"
 	"sort"
@@ -113,7 +114,27 @@ func run(cli cliFlags) error {
 		return fmt.Errorf("seed random pool: %w", err)
 	}
 
-	httpClient := &http.Client{Timeout: cli.timeout}
+	// Bump per-host idle connections so concurrent goroutines reuse
+	// keep-alive sockets instead of churning TCP+TLS handshakes —
+	// stdlib default MaxIdleConnsPerHost=2 caps in-flight requests
+	// to 2 keep-alive sockets per target, which serializes any
+	// concurrency>2 onto fresh connections each request.
+	transport := http.DefaultTransport.(*http.Transport).Clone()
+	transport.MaxIdleConns = 2 * cli.concurrency
+	transport.MaxIdleConnsPerHost = 2 * cli.concurrency
+	transport.MaxConnsPerHost = 2 * cli.concurrency
+	httpClient := &http.Client{Timeout: cli.timeout, Transport: transport}
+
+	// pprof on a dedicated listener — `_ "net/http/pprof"` registers
+	// handlers on http.DefaultServeMux. Always-on at 127.0.0.1:6060
+	// since this is a load-tester binary, not a production daemon;
+	// SSH port-forward to grab profiles under load:
+	//
+	//	ssh -L 6060:127.0.0.1:6060 root@loadgen \
+	//	  go tool pprof http://localhost:6060/debug/pprof/profile?seconds=10
+	go func() {
+		_ = http.ListenAndServe("127.0.0.1:6060", nil)
+	}()
 
 	ctx, cancel := context.WithCancel(context.Background())
 	if cli.duration > 0 {