fix: resolve exported type stuttering issues (revive)

- Rename VaultMetadata to Metadata in internal/vault package to avoid stuttering
- Rename BIP85DRNG to DRNG in pkg/bip85 package to avoid stuttering
- Update all references in code and tests

internal/vault/metadata_test.go

@@ -68,7 +68,7 @@ func TestVaultMetadata(t *testing.T) {
 			t.Fatalf("Failed to write public key: %v", err)
 		}
 
-		metadata1 := &VaultMetadata{
+		metadata1 := &Metadata{
 			DerivationIndex:    0,
 			PublicKeyHash:      pubKeyHash0, // Hash of the actual key (index 0)
 			MnemonicFamilyHash: pubKeyHash0, // Hash of index 0 key (for family identification)
@@ -117,7 +117,7 @@ func TestVaultMetadata(t *testing.T) {
 		// Compute the hash for index 5 key
 		pubKeyHash5 := ComputeDoubleSHA256([]byte(pubKey5))
 
-		metadata2 := &VaultMetadata{
+		metadata2 := &Metadata{
 			DerivationIndex:    5,
 			PublicKeyHash:      pubKeyHash5, // Hash of the actual key (index 5)
 			MnemonicFamilyHash: pubKeyHash0, // Same family hash since it's from the same mnemonic
@@ -143,7 +143,7 @@ func TestVaultMetadata(t *testing.T) {
 		}
 
 		// Create and save metadata
-		metadata := &VaultMetadata{
+		metadata := &Metadata{
 			DerivationIndex: 3,
 			PublicKeyHash:   "test-public-key-hash",
 		}