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>

BUGS.md

@@ -102,21 +102,6 @@ decisions, see `docs/limiti-noti.md`):
 
 ## Critical
 
-### B-79 — A payout normally has no change output, so it cannot be fee-bumped
-
-The pool normally holds exactly the round's bet outputs, the payout spends them all
-for exactly `pool_amount_sats`, so there is no change and `bump_fee` can never help.
-A payout stuck in a mempool is also never abandoned (the server still knows it): the
-round stays in `paying_out` and no new round opens.
-
-Fix: every payout carries a **bump reserve** output of
-`max(DUST_LIMIT_SATS, estimated_vsize × 10 sat/vB)`, taken from the commission share
-and paid to the pool reserve address (`m/84'/746'/0'/1/0`, the old pool address);
-RBF shrinks that output. `rounds_can_open` refuses a configuration in which a
-round of 1 or of `MAX_PARTICIPANTS_PER_ROUND` participants could not cover winner
-fee + reserve + dust commission, and `PUT /admin/config` refuses a bet amount or
-fee rate that would fail the same check (spec: RND-40).
-
 ### B-82 — A deposit reorged out after a bet leaves the round unpayable
 
 Deposits are credited at 1 confirmation. A user can bet, then (if they lost)