Cardano Updates

Per @carlostome's review of #1161, move CIP-159 direct-deposit
application out of the LEDGER/SUBLEDGER orchestration rules and into
the POST-CERT sub-rule of CERTS, alongside the existing voteDelegs
filtering.

CertEnv gains a sixth field, directDeposits, populated by LEDGER-V
(DirectDepositsOf tx) and SUBLEDGER-V (DirectDepositsOf stx) when
constructing each CERTS invocation's environment.  POST-CERT (CERT-post)
gains a premise dom directDeposits ⊆ dom rewards and updates its
post-state's rewards to rewards ∪⁺ directDeposits.

SUBLEDGER-V and LEDGER-V revert to threading-only: certStateWithDDeps,
the certStateFinal let-binding, and the dom-check premises are removed.
Computational instances are updated correspondingly.

Per-transaction semantics are preserved (POST-CERT runs at the end of
each transaction's CERTS invocation).  Phantom-asset prevention via
NoPhantomWithdrawals is unaffected (accountBalances is fixed at the
pre-batch snapshot).  depositsChange remains orthogonal
(applyDirectDeposits touches rewards only; calculateDepositsChange
reads deposits only).

Direct-deposit documentation moves from Ledger.lagda.md to the POST-CERT
section of Certs.lagda.md.

checkPartialFanout now verifies that headInValue equals headOutValue plus the sum of all distributed output values, preventing Ada theft via outputs not counted by the membership proof. The off-chain partialFanoutTx builder is updated accordingly to subtract distributed values from the continuing head output.