switch to math/rand/v2 in most places

this allows removing some ugly instantiations of an rng based on the current
time.

Intn is now IntN for our concurrency-safe prng wrapper to match the randv2 api.

v2 exists since go1.22, which we already require.

mox-/rand.go

@@ -4,19 +4,23 @@ import (
 	cryptorand "crypto/rand"
 	"encoding/binary"
 	"fmt"
-	mathrand "math/rand"
+	mathrand2 "math/rand/v2"
 	"sync"
 )
 
 type rand struct {
-	rand *mathrand.Rand
+	rand *mathrand2.Rand
 	sync.Mutex
 }
 
 // NewPseudoRand returns a new PRNG seeded with random bytes from crypto/rand. Its
 // functions can be called concurrently.
 func NewPseudoRand() *rand {
-	return &rand{rand: mathrand.New(mathrand.NewSource(CryptoRandInt()))}
+	var seed [32]byte
+	if _, err := cryptorand.Read(seed[:]); err != nil {
+		panic(err)
+	}
+	return &rand{rand: mathrand2.New(mathrand2.NewChaCha8(seed))}
 }
 
 func (r *rand) Float64() float64 {
@@ -25,16 +29,10 @@ func (r *rand) Float64() float64 {
 	return r.rand.Float64()
 }
 
-func (r *rand) Intn(n int) int {
+func (r *rand) IntN(n int) int {
 	r.Lock()
 	defer r.Unlock()
-	return r.rand.Intn(n)
-}
-
-func (r *rand) Read(buf []byte) (int, error) {
-	r.Lock()
-	defer r.Unlock()
-	return r.rand.Read(buf)
+	return r.rand.IntN(n)
 }
 
 // CryptoRandInt returns a cryptographically random number.