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.

Nixpkgs expects to drop support for intel macs in 26.11.