tun: default pin_threads off

Thread pinning trades scheduler freedom for TX-ring ordering; that's the
right trade on dedicated forwarders but not as a surprise default on
hosts sharing cores with other workloads. Make it opt-in and document
the default in the example config.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

main.go

@@ -236,7 +236,7 @@ func Main(c *config.C, configTest bool, buildVersion string, l *slog.Logger, dev
 		punchy:                punchy,
 		ConntrackCacheTimeout: conntrackCacheTimeout,
 		CpuAffinity:           parseCpuAffinity(c, l, routines),
-		PinThreads:            c.GetBool("tun.pin_threads", true),
+		PinThreads:            c.GetBool("tun.pin_threads", false),
 		l:                     l,
 	}