Cardano Updates

- Add Cardano.Balance.Tx.Primitive re-exporting coin-selection types
  (Coin, TokenBundle, TokenMap) and defining local types (TxSize,
  Address, TxIn, TxOut, UTxO) with Buildable instances
- Add Cardano.Balance.Tx.Primitive.Convert with all ledger conversion
  functions (toLedgerCoin, fromLedgerCoin, toLedger/fromLedger for
  TokenBundle, TxIn, Address, TxOut, UTxO, PolicyId, AssetName,
  and Timelock scripts)
- Update all library modules to import from Primitive instead of
  cardano-wallet-primitive
- Inline conflictsWith from Data.Map.Strict.Extra into Balance.hs
- Inline QuickCheck.Extra helpers into TxWithUTxO/Gen.hs
- Remove cardano-wallet-primitive and cardano-wallet-test-utils from
  library build-depends
- Remove wallet source-repository-package from cabal.project (library
  has zero wallet dependencies)
- Add iohk-nix overlays to flake.nix for crypto libs (libblst,
  libsodium-vrf, secp256k1)
- Add pkgconfig module overrides in nix/project.nix
- Temporarily disable test suite (pending Task #5)

Library builds successfully with GHC 9.12.2 — all 16 modules compile.

Copy lib/balance-tx/ from cardano-wallet PR #5193 branch.
Rename module namespace: Internal.Cardano.Write.* → Cardano.Balance.Tx.*
Add cabal.project with CHaP and wallet source-repository-packages.

Wallet dependencies kept temporarily — to be removed iteratively.

Nix flake, CI (Cachix), release-please, fourmolu, hlint,
mkdocs, justfile — mirroring cardano-coin-selection setup.