Give every payout a bump reserve output and refuse configs that cannot pay a round (B-79, RND-21, RND-23, RND-24, RND-27, RND-38, RND-40, DAT-73, API-31, API-36, TX-11, ADR-17, ADR-29)

The pool normally holds exactly the round's bet outputs and the payout spent them all for
exactly the pool amount, so it had no change output: bump_fee could never accelerate it,
and a payout stuck in a mempool (never abandoned while a server knows it) kept the round in
`paying_out` and the whole lottery stopped.

- build_payout_transaction always has three outputs: winner, fee address, and a bump reserve
  of max(DUST_LIMIT_SATS, estimate_vsize(inputs, 3) * 10 sat/vB) paid to the reserve address
  m/84'/746'/0'/1/0. The reserve comes out of the platform's 30%, the network fee still out
  of the winner's 70%. Input value beyond winner share + commission (only rounds opened
  before the per-round pool address) is folded into the reserve output if >= dust, left to
  the network fee otherwise. `change_address` is replaced by `reserve_address`.
- `rounds.bump_reserve_sats` (Alembic e5a91c3d7b26, plain add_column) is written with the
  payout intent before the broadcast; /admin/rounds returns it.
- bump_fee takes the increase from the reserve output for a payout
  (_find_reducible_output; a payout already in flight with a change to the pool still falls
  back to that), never from the winner or fee-address outputs.
- payout_config_problem / rounds_can_open (RND-40): a bet amount and fee rate under which a
  round of 1 or of MAX_PARTICIPANTS_PER_ROUND participants could not cover the network fee,
  the reserve and dust outputs opens no round (lottery_configured=false); PUT /admin/config
  refuses such a bet amount or fee rate with 422 config_not_payable, judged on the resulting
  values, since the payout reads the fee rate live.

Ambiguity resolved: the reserve output and any excess input are one output (RND-23 says one
output to the reserve address), and bump_reserve_sats records that output's initial value.
DAT-22's "abandon clears all amounts" stays with B-96.

Tests changed because the spec changed: the four test_payout_builder tests asserted the old
two/three-output shape with change to the pool and the fee address receiving the whole 30%;
the whitespace fee_address test now spells out the amounts rounds_can_open reads. New: reserve
sizing, payout with 1 and with 400 participants (inputs = outputs + fee to the satoshi),
exact commission/reserve/dust boundary, excess folded or left to the fee, a real payout bumped
out of its reserve with the other outputs byte-identical and bumped five times, the old
reserve-less shape still raising RbfError, the scheduler output addresses and recorded
amounts, payability boundaries found independently by bisection, no round / unconfigured flag
for an unpayable config, a round in progress surviving it, PUT /admin/config refusals and the
migration column. Mutation-checked: dropping the reserve lookup in bump_fee and the payability
check in rounds_can_open each fail tests.

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

app/tx/broadcast.py

@@ -23,7 +23,7 @@ from app.electrum.client import ElectrumClient
 from app.rounds.config import get_round_config
 from app.tx.outcome import BroadcastOutcome, TransactionNetwork, broadcast_with_verdict
 from app.tx.versions import record_version
-from app.wallet.hd import derive_round_pool_key, derive_user_key
+from app.wallet.hd import derive_pool_address, derive_round_pool_key, derive_user_key
 from app.wallet.plm_network import PLM_MAINNET
 from app.wallet.psbt_builder import MAX_FEE_RATE_SAT_VB, RBF_SEQUENCE, estimate_vsize
 
@@ -102,6 +102,19 @@ def _find_change_output(tx: Transaction, change_address: str) -> int | None:
     return None
 
 
+def _find_reducible_output(tx: Transaction, kind: str, own_address: str) -> int | None:
+    """The one output an RBF bump takes the extra fee from (TX-11): the sender's own change
+    for a bet or a withdrawal; for a payout the **bump reserve** output paid to the reserve
+    address (B-79, RND-38), falling back to a change output back to the pool for a payout
+    that was already in flight before the reserve existed. Counterparty outputs — the
+    winner's, the fee address's, an external recipient's — are never candidates."""
+    if kind == "payout":
+        reserve_index = _find_change_output(tx, derive_pool_address())
+        if reserve_index is not None:
+            return reserve_index
+    return _find_change_output(tx, own_address)
+
+
 async def bump_fee(
     session_factory: async_sessionmaker,
     client: ElectrumClient,
@@ -179,7 +192,7 @@ async def bump_fee(
     min_valid_delta = vsize * _INCREMENTAL_RELAY_FEE_RATE_SAT_VB
     fee_delta = max(target_fee - old_fee, min_valid_delta)
 
-    change_index = _find_change_output(old_tx, own_address)
+    change_index = _find_reducible_output(old_tx, kind, own_address)
     if change_index is None or old_tx.vout[change_index].value <= fee_delta:
         raise RbfError(f"pending_transaction {pending_id}: no change output large enough to absorb a fee bump")