Cardano Updates

For consistency with SUBUTXOW

After direct-deposit application moved from LEDGER-V/SUBLEDGER-V into the
POST-CERT rule, the Certs preservation-of-value proofs needed to account for
the `getCoin (DirectDepositsOf Γ)` increase that POST-CERT now produces via
`rewards ∪⁺ directDeposits`.

Statement changes:
- POST-CERT-pov:  getCoin s ≡ getCoin s'
              →  getCoin s + getCoin (DirectDepositsOf Γ) ≡ getCoin s'
- sts-pov:        gains a `+ getCoin (DirectDepositsOf Γ)` term on the LHS
- CERTS-pov:      becomes the symmetric "consumed = produced" form
                  getCoin s₁ + getCoin (DirectDepositsOf Γ)
                  ≡ getCoin sₙ + getCoin (WithdrawalsOf Γ)

Structural changes:
- POST-CERT-pov and sts-pov move into the parameterized `Certs-Pov-lemmas`
  sub-module (alongside PRE-CERT-pov), since they now require a fourth
  module parameter:
    indexedSumᵛ'-∪⁺ : ∀ (m m' : Rewards) → getCoin (m ∪⁺ m') ≡ getCoin m + getCoin m'
  This is the natural ∪⁺ analogue of the existing `indexedSumᵛ'-∪` lemma for
  `∪ˡ` on disjoint domains, but unconditional because `∪⁺` adds (rather than
  drops) values at shared keys.  TODO: upstream to agda-sets.
- `Certs-PoV` (in PoV.lagda.md) gains the same parameter and forwards it.

CERT-pov and PRE-CERT-pov are unchanged: the CERT and PRE-CERT rules did
not change in this refactor.

Closes part of #1185.