lint: fix remaining 3 math.Mul64 (secp256k1fx/secp256r1fx/slhdsafx)

golangci-lint's max-same-issues:3 truncated the CI display to 3 of 6 Mul64
sites; v0.5.2 fixed the 3 shown, these are the 3 suppressed ones. All safemath
Mul64/Add64 now use the generic math.Mul/Add. CI lint job now clean (the 3
residual staticcheck findings are Go-1.26 crypto-coordinate deprecations in
secp256r1fx/keychain.go that golangci-lint v2.1.6 does not flag; crypto/ecdh
refactor tracked separately). 11/11 packages pass.

Co-authored-by: Hanzo Dev <dev@hanzo.ai>

secp256k1fx/input.go

@@ -28,7 +28,7 @@ type Input struct {
 
 func (in *Input) Cost() (uint64, error) {
 	numSigs := uint64(len(in.SigIndices))
-	return math.Mul64(numSigs, CostPerSignature)
+	return math.Mul(numSigs, CostPerSignature)
 }
 
 // Verify this input is syntactically valid

secp256r1fx/input.go

@@ -29,7 +29,7 @@ type Input struct {
 
 func (in *Input) Cost() (uint64, error) {
 	numSigs := uint64(len(in.SigIndices))
-	return math.Mul64(numSigs, CostPerSignature)
+	return math.Mul(numSigs, CostPerSignature)
 }
 
 // Verify this input is syntactically valid

slhdsafx/input.go

@@ -29,7 +29,7 @@ type Input struct {
 
 func (in *Input) Cost() (uint64, error) {
 	numSigs := uint64(len(in.SigIndices))
-	return math.Mul64(numSigs, CostPerSignature)
+	return math.Mul(numSigs, CostPerSignature)
 }
 
 // Verify this input is syntactically valid