…veForwardBlock drain loss

Adds TestMoveForwardBlockDrainLoss_BatchesLostOnPostDrainError - a
deterministic, tests-only reproduction of the rollback gap described in
issue bsv-blockchain#852.

Setup:

* errOnCreateUtxoStore wraps a real sqlitememory utxo store via
  interface embedding and overrides Create to return a sentinel error.
  All other methods delegate to the embedded store. This injects a
  step-6 failure (processCoinbaseUtxos -> utxoStore.Create) AFTER
  step-5 has already drained the queue.
* Clocks pinned via fixedClock so DoubleSpendWindow=0 admits the
  enqueued batch at drain time (consistent with the bsv-blockchain#846 fix).
* SubtreeProcessor built without Start so the event-loop goroutine
  cannot drain the batch concurrently. moveForwardBlock is invoked
  directly (lowercase) and the production rollback (lines 711-717) is
  replayed inline by the test.

What the test demonstrates:

1. Pre-call: queue contains one enqueued batch.
2. moveForwardBlock errors with the sentinel from the Create wrapper.
3. After the inline rollback, the snapshotted in-memory fields
   (chainedSubtrees, currentSubtree, currentTxMap, currentBlockHeader)
   are restored.
4. The queue is empty: processRemainderTransactionsAndDequeue drained
   the batch into the now-discarded new subtree state.
5. The drained tx hash is in neither the queue nor any subtree -> the
   batch is silently lost.

This test PINS the current buggy behaviour. When the underlying fix
lands (option 1, 2, or 3 from the issue), the assertion at queue
length 0 will need to flip to require.Equal(t, preLen, postLen) and
require.Contains for the tx hash in the queue.

Test plan:
- go vet ./services/blockassembly/subtreeprocessor/ - clean
- go test -race -count=1 -run TestMoveForwardBlockDrainLoss ./services/blockassembly/subtreeprocessor/ - pass (~7s)
- go test -race -count=1 ./services/blockassembly/subtreeprocessor/ - pass (~155s)

Refs bsv-blockchain#852.

…entation

gofmt/gci flagged the numbered list in the repro test's doc comment for
non-canonical indentation (3-space vs 2-space). Reformat to gofmt's
canonical list shape so CI passes.