Replace TODO("Not yet implemented") with an explicit UnsupportedOperationException

subscribeAccountBalanceUpdatePoint() on Bitcoin/Tron-native/TRC20/EVM-native
balance sources is only ever called from RealCrossChainTransactor's XCM
arrival detection, which is Substrate-only - none of these chain families
can be an XCM cross-chain destination, so the method is intentionally never
reachable. A bare TODO() reads as forgotten/unfinished work; swapping it for
the same UnsupportedOperationException pattern UnsupportedAssetBalance
already uses elsewhere in this codebase makes the same never-called
guarantee explicit and documented instead of implicit.

feature-wallet-impl/src/main/java/io/novafoundation/nova/feature_wallet_impl/data/network/blockchain/assets/balances/bitcoinNative/BitcoinNativeAssetBalance.kt

@@ -59,8 +59,9 @@ class BitcoinNativeAssetBalance(
         chainAsset: Chain.Asset,
         accountId: AccountId,
     ): Flow<TransferableBalanceUpdatePoint> {
-        // Not on the critical sync path (mirrors TronNativeAssetBalance/EvmNativeAssetBalance) - out of scope for Phase 4.
+        // Only ever invoked from RealCrossChainTransactor (XCM arrival detection), which is Substrate-only -
-        TODO("Not yet implemented")
+        // Bitcoin can never be an XCM cross-chain destination, so this is intentionally never reachable.
+        throw UnsupportedOperationException("Bitcoin does not support XCM-style balance update points")
     }
 
     override suspend fun startSyncingBalance(

feature-wallet-impl/src/main/java/io/novafoundation/nova/feature_wallet_impl/data/network/blockchain/assets/balances/evmNative/EvmNativeAssetBalance.kt

@@ -63,7 +63,10 @@ class EvmNativeAssetBalance(
         chainAsset: Chain.Asset,
         accountId: AccountId,
     ): Flow<TransferableBalanceUpdatePoint> {
-        TODO("Not yet implemented")
+        // Only ever invoked from RealCrossChainTransactor (XCM arrival detection), which is Substrate-only -
+        // an EVM chain's native asset can never be an XCM cross-chain destination, so this is intentionally
+        // never reachable.
+        throw UnsupportedOperationException("EVM native assets do not support XCM-style balance update points")
     }
 
     override suspend fun startSyncingBalance(

feature-wallet-impl/src/main/java/io/novafoundation/nova/feature_wallet_impl/data/network/blockchain/assets/balances/trc20/Trc20AssetBalance.kt

@@ -62,8 +62,9 @@ class Trc20AssetBalance(
         chainAsset: Chain.Asset,
         accountId: AccountId,
     ): Flow<TransferableBalanceUpdatePoint> {
-        // Not on the critical sync path (mirrors EvmNativeAssetBalance) - out of scope for Phase 1 read-only support.
+        // Only ever invoked from RealCrossChainTransactor (XCM arrival detection), which is Substrate-only -
-        TODO("Not yet implemented")
+        // Tron/TRC20 can never be an XCM cross-chain destination, so this is intentionally never reachable.
+        throw UnsupportedOperationException("TRC20 does not support XCM-style balance update points")
     }
 
     override suspend fun startSyncingBalance(

feature-wallet-impl/src/main/java/io/novafoundation/nova/feature_wallet_impl/data/network/blockchain/assets/balances/tronNative/TronNativeAssetBalance.kt

@@ -59,9 +59,9 @@ class TronNativeAssetBalance(
         chainAsset: Chain.Asset,
         accountId: AccountId,
     ): Flow<TransferableBalanceUpdatePoint> {
-        // Not on the critical sync path (mirrors EvmNativeAssetBalance, which also leaves this unimplemented) -
+        // Only ever invoked from RealCrossChainTransactor (XCM arrival detection), which is Substrate-only -
-        // out of scope for Phase 1 read-only support.
+        // Tron can never be an XCM cross-chain destination, so this is intentionally never reachable.
-        TODO("Not yet implemented")
+        throw UnsupportedOperationException("Tron does not support XCM-style balance update points")
     }
 
     override suspend fun startSyncingBalance(