chore: name the bridge a transfer charged its fee in

internal/migrations/erc20-bridge/002-public-transfer-actions.prod.sql

@@ -15,6 +15,11 @@
 -- fee is paid in the SAME bridge as the transfer (not always in TRUF).
 -- This avoids forcing USDC senders to also hold TRUF.
 --
+-- Because of that, and because transaction_events carries no token column, both
+-- actions record the bridge in the ledger event's metadata. Without it the two
+-- fees are indistinguishable in the ledger and a reader has to infer the token
+-- from the amount, which only works while the two fees stay different constants.
+--
 -- Manual-apply mainnet override. The embedded migration loader skips
 -- *.prod.sql, so apply via:
 --
@@ -58,11 +63,14 @@ CREATE OR REPLACE ACTION eth_truf_transfer($to_address TEXT, $amount TEXT) PUBLI
   -- Execute transfer using the bridge extension
   eth_truf.transfer($to_address, $amount::NUMERIC(78, 0));
 
+  -- The fee is paid in the bridge that moved, not always in $TRUF, and
+  -- transaction_events has no token column. Naming the bridge here is what lets
+  -- a reader tell this row's 1 TRUF from eth_usdc_transfer's 1 USDC.
   record_transaction_event(
     4,
     $fee,
     '0x' || $leader_hex,
-    NULL
+    '{"bridge":"eth_truf"}'
   );
 };
 
@@ -100,10 +108,12 @@ CREATE OR REPLACE ACTION eth_usdc_transfer($to_address TEXT, $amount TEXT) PUBLI
   -- Execute transfer using the bridge extension
   eth_usdc.transfer($to_address, $amount::NUMERIC(78, 0));
 
+  -- See eth_truf_transfer above: without this the 6-decimal fee is
+  -- indistinguishable from an 18-decimal one in the ledger.
   record_transaction_event(
     4,
     $fee,
     '0x' || $leader_hex,
-    NULL
+    '{"bridge":"eth_usdc"}'
   );
 };

internal/migrations/erc20-bridge/002-public-transfer-actions.sql

@@ -37,7 +37,7 @@ CREATE OR REPLACE ACTION sepolia_transfer($to_address TEXT, $amount TEXT) PUBLIC
     4,
     $fee,
     '0x' || $leader_hex,
-    NULL
+    '{"bridge":"sepolia_bridge"}'
   );
 };
 
@@ -78,7 +78,7 @@ CREATE OR REPLACE ACTION ethereum_transfer($to_address TEXT, $amount TEXT) PUBLI
     4,
     $fee,
     '0x' || $leader_hex,
-    NULL
+    '{"bridge":"ethereum_bridge"}'
   );
 };
 
@@ -119,7 +119,7 @@ CREATE OR REPLACE ACTION hoodi_tt_transfer($to_address TEXT, $amount TEXT) PUBLI
     4,
     $fee,
     '0x' || $leader_hex,
-    NULL
+    '{"bridge":"hoodi_tt"}'
   );
 };
 
@@ -160,6 +160,6 @@ CREATE OR REPLACE ACTION hoodi_tt2_transfer($to_address TEXT, $amount TEXT) PUBL
     4,
     $fee,
     '0x' || $leader_hex,
-    NULL
+    '{"bridge":"hoodi_tt2"}'
   );
 };

tests/streams/transaction_events_ledger_test.go

@@ -204,6 +204,19 @@ func runTransactionEventsLedgerScenario(t *testing.T) func(ctx context.Context,
 		require.NoError(t, err)
 		height++
 
+		// A second transfer on a different bridge. Two transfer actions booking
+		// two different bridges under the same method is the whole point of
+		// recording one: the amount alone cannot tell them apart once a bridge
+		// with different decimals exists. The actor is already funded on
+		// hoodi_tt for the write fees above, so this needs no extra setup.
+		ttTransferLeaderPub, ttTransferLeaderAddr := newLeader(t)
+		ttTransferTx, err := callActionWithLeader(ctx, platform, actor, ttTransferLeaderPub, height, "hoodi_tt_transfer", []any{
+			receiver.Address(),
+			transferAmount,
+		})
+		require.NoError(t, err)
+		height++
+
 		withdrawLeaderPub, withdrawLeaderAddr := newLeader(t)
 		withdrawTx, err := callActionWithLeader(ctx, platform, actor, withdrawLeaderPub, height, "sepolia_bridge_tokens", []any{
 			actor.Address(),
@@ -238,6 +251,17 @@ func runTransactionEventsLedgerScenario(t *testing.T) func(ctx context.Context,
 			require.Empty(t, meta, "expected no metadata for ledger event")
 		}
 
+		// A transfer is the one method that does not always charge its fee in
+		// $TRUF: it charges in the bridge it moves. The ledger has no token
+		// column, so unless each action names its own bridge a reader cannot
+		// tell a 1 TRUF fee from a 1 USDC one, and the two differ by 1e12.
+		assertBridge := func(want string) func(meta metadataMap) {
+			return func(meta metadataMap) {
+				require.Equal(t, want, meta.String("bridge"),
+					"a transfer must name the bridge its fee was charged in")
+			}
+		}
+
 		expected := map[string]ledgerExpectation{
 			createTx: {
 				method:       "deployStream",
@@ -274,7 +298,16 @@ func runTransactionEventsLedgerScenario(t *testing.T) func(ctx context.Context,
 				feeDistributions: []string{
 					buildDistribution(transferLeaderAddr, feeOneTRUF),
 				},
-				assertMetadata: assertNoMetadata,
+				assertMetadata: assertBridge("sepolia_bridge"),
+			},
+			ttTransferTx: {
+				method:       "transferTN",
+				fee:          feeOneTRUF,
+				feeRecipient: ttTransferLeaderAddr,
+				feeDistributions: []string{
+					buildDistribution(ttTransferLeaderAddr, feeOneTRUF),
+				},
+				assertMetadata: assertBridge("hoodi_tt"),
 			},
 			withdrawTx: {
 				method:       "withdrawTN",