Introduce shadow tickets for export replay protection:
- ltSHADOW_TICKET ledger entry: account-owned, keyed by
account + ticket sequence. Fields: sfAccount, sfTicketSequence,
sfTransactionHash, sfLedgerSequence, sfOwnerNode.
- ExportLedgerOps::createShadowTicket(): creates shadow ticket
when exported tx has sfTicketSequence. Charges owner reserve.
Called from both hook xport() path and Export transactor.
- ExportLedgerOps::cancelShadowTicket(): deletes shadow ticket,
frees reserve. Used by xport_cancel hook API.
- xport_cancel(ticket_seq) hook API: allows hooks to cancel
shadow tickets for exports that will never get a callback.
- InvariantCheck: add ltSHADOW_TICKET to valid entry types.
- Test: verify shadow ticket creation with correct fields and
owner count bump via ttEXPORT with TicketSequence.
Resolve 14 conflicts keeping both sides. Renumber TOO_LITTLE_ENTROPY
from -46 to -48 to avoid collision with export error codes.
Fix sfHookExportCount to soeOPTIONAL in InnerObjectFormats (only set
when featureExportRNG is enabled).
