payout: key obligations on block hash, and wait for coin instead of expiring

A 280-block reorg (heights 3075-3354) orphaned 292 of the pool's 707 blocks
and destroyed 14,600 BTF of coinbase. The payout ledger did not notice: it
records only matureAtHeight, and a reorg replaces the block at a height
without changing the height, so ~14.4k BTF of obligations outlived the coin
that was supposed to fund them. The pool then paid fork-era miners out of
real-chain coin until the wallet was empty -- 17,827 BTF paid against 17,900
BTF of matured supply, leaving under 1.1 BTF spendable and every subsequent
payment failing.

Three changes, all in the payout path:

- Record the funding block's hash and height, persist both, and void an
  obligation whose block is no longer in the main chain. Checked both at
  maturity and again in the exec queue, since an entry can sit there for
  hours across retries and the reorg can land at any point in that window.
  Entries loaded from a file written before this are grandfathered rather
  than voided on a missing field.

- Stop spending the retry budget on maturity. SelectCoins failing for want
  of spendable coin is not an error, but it consumed one of 1440 retries
  every 60s, so a payment the pool fully intended to honour would hit
  GIVING UP 24h later and the miner would silently never be paid. Those
  attempts now defer without incrementing retries, logged hourly rather
  than every minute. GIVING UP now means what it says: 1440 real failures
  against a block still on-chain with the money available.

- Schedule at COINBASE_SPENDABLE_DEPTH instead of COINBASE_MATURITY. At
  depth 100 a block's own coinbase is 20 blocks short of spendable, so its
  payout had to come from unrelated mature coin and the pool carried a
  permanent ~19-block float. The "96% of 934 payments went through at depth
  100" note that justified the shorter wait was measured while this node was
  alone on a private fork paying itself. The extra 20 blocks also put the
  maturity gate past the reorg that caused all of the above.

(cherry picked from commit 696386220a3d2fac88cb8045931392f4909e0985)

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