math/rand/v2: add method Rand.N

Derived from CL 775100 by qiu laidongfeng <2645477756@qq.com>

Fixes #77853

Change-Id: Iefcdbd1a7dbf82e732dad9bbf94722878c82c1a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/780582
Reviewed-by: qiu laidongfeng <2645477756@qq.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Austin Clements <austin@google.com>

doc/next/6-stdlib/99-minor/math/rand/v2/77853.md

@@ -0,0 +1 @@
+add the generic method [*Rand].N, matching the behavior of the top-level N function.

src/math/rand/v2/n.go

@@ -0,0 +1,21 @@
+//go:build goexperiment.genericmethods
+
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+package rand
+
+// N returns a pseudo-random number in the half-open interval [0,n).
+// The type parameter Int can be any integer type.
+// It panics if n <= 0.
+func (r *Rand) N[Int intType](n Int) Int {
+	// TODO: See CL 775100,
+	// When delete the goexperiment.genericmethods,
+	// make this method
+	// share the implementation with the global function N
+	// and delete this file.
+	if n <= 0 {
+		panic("invalid argument to N")
+	}
+	return Int(r.uint64n(uint64(n)))
+}