Never roll back an ambiguous broadcast as if it had been refused (B-76, TX-04, TX-05, TX-20, TX-21, BET-16, BET-17, BET-23, WDR-11, WDR-12, WDR-16, API-13)

place_bet and request_withdrawal released everything on any exception from the
broadcast, including the 15s timeout and a connection dropped after the write. The
tx may have propagated: a bet's sats landed in the pool with no participant, a
withdrawal left while its row said failed and its coins were spendable again.

- client: ElectrumRpcError (subclass of ElectrumError) is raised only for a reply
  carrying a JSON-RPC `error`; timeouts, drops and write errors stay plain
  ElectrumError.
- tx/outcome.py:broadcast_with_verdict sorts a broadcast into accepted / refused /
  uncertain. A refusal is verified first: ElectrumListener.transaction_known asks
  blockchain.transaction.get of the active server and every other one (10s each) —
  any server returning the tx means accepted; the active server plus
  MIN_QUORUM_RESPONSES others all denying it means refused (ALLOW_NO_QUORUM keeps
  the reduced rule); too few answers is uncertain. "already known" wording is only a
  shortcut that skips the check. With no network to ask, a refusal is uncertain.
- place_bet / request_withdrawal / _trigger_payout use it: only a verified refusal
  releases; uncertain leaves the rows building for the reconciler, publishes SSE and
  POST /bets and POST /withdrawals answer 202 with status building; "already known"
  promotes a payout (payout_sent) like any accepted broadcast.
- An accepted transaction is fanned out to every other server
  (ElectrumListener.fan_out_transaction, best effort, off the caller's path). The RBF
  replacement in bump_fee is left for B-77, which rewrites that function.

Tests changed because the spec changed: the existing "refused broadcast" tests raised
a bare RuntimeError (now an uncertain outcome) and are made explicit refusals
(rpc_error) with a network that denies the tx; direct route calls pass a Response;
FakePayoutListener gains the network surface.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

app/electrum/client.py

@@ -128,6 +128,20 @@ class ElectrumError(Exception):
     pass
 
 
+class ElectrumRpcError(ElectrumError):
+    """The server *answered* with a JSON-RPC `error` object (B-76, TX-04). It is the
+    only failure that says anything about the request itself: a timeout, a dropped
+    connection or a write error (plain ElectrumError) leaves it unknown whether the
+    server ever acted on it. `args[0]` is the error object as the server sent it."""
+
+    @property
+    def message(self) -> str:
+        detail = self.args[0] if self.args else ""
+        if isinstance(detail, dict):
+            return str(detail.get("message", detail))
+        return str(detail)
+
+
 class ElectrumClient:
     """Minimal asyncio Electrum protocol client: line-delimited JSON-RPC over TLS.
 
@@ -266,6 +280,13 @@ class ElectrumClient:
     async def get_transaction(self, txid: str, verbose: bool = False) -> object:
         return await self.request("blockchain.transaction.get", [txid, verbose])
 
+    async def get_merkle(self, txid: str, height: int) -> dict:
+        """The merkle branch proving `txid` is in the block at `height` (B-83, ELC-26)."""
+        return await self.request("blockchain.transaction.get_merkle", [txid, height])
+
+    async def get_block_header(self, height: int) -> str:
+        return await self.request("blockchain.block.header", [height])
+
     async def _read_loop(self) -> None:
         """Frame the stream into lines by hand rather than with StreamReader.readline:
         readline raises on a line over the limit, and that exception used to end this
@@ -350,7 +371,7 @@ class ElectrumClient:
             if future.done():
                 return
             if message.get("error"):
-                future.set_exception(ElectrumError(message["error"]))
+                future.set_exception(ElectrumRpcError(message["error"]))
             else:
                 future.set_result(message.get("result"))
         elif "method" in message: