Cardano Updates

  (cardano-ledger#5360)

Two fixes for goroutine deadlocks that silently killed the chainsync
pipeline, causing nodes to stop advancing with no error output.

1. Close result channel on reader exit: ledgerReadChain() could exit
   on errors (chain reconcile failure, block decode, nil iterator)
   without closing resultCh. The consumer blocked forever on the
   channel read. Add defer close(resultCh) so the consumer always
   unblocks and the caller can create a fresh reader.

2. Preserve done channel for cached batches: when an epoch rollover
   split a batch, the next loop iteration reset currentReadResultDone
   to nil before consuming the cached remainder. The reader goroutine
   blocked forever waiting for its done channel to close. Move the
   nil reset inside the else branch so it only runs when reading
   fresh from the channel.

Signed-off-by: wcatz <[email protected]>
Signed-off-by: wcatz <[email protected]>

Use safeAddUint64(K, K) instead of 2*K to prevent uint64 overflow
when securityParam is large.

Signed-off-by: wcatz <[email protected]>
Signed-off-by: wcatz <[email protected]>