Migrate tx-generator to the experimental Cardano API (Conway and later)

## Background

Raised by @Jimbo4350 in review of PR #6575.

The experimental Cardano API (`Cardano.Api.Experimental`) provides `makeUnsignedTx` and `signTx` as the supported transaction construction path, replacing direct ledger-level construction.
It only supports Conway and Dijkstra eras.

Mainnet is on Conway (protocol version 9+) and all production benchmark profiles already target Conway exclusively.
No active profile sets an era below Conway.
The pre-Conway code paths are dead in production.

This targets the master branch version of the code.

## Scope

Paths are relative to `bench/tx-generator/`.

### Core transaction building

- [ ] **`src/Cardano/TxGenerator/Tx.hs`** - rewrite `genTx` from manual `Ledger.mkBasicTx` / `WitVKey` / `ShelleyTx` construction to `Exp.makeUnsignedTx` + `Exp.makeKeyWitness` + `Exp.signTx`.

### Era dispatch - narrow to Conway + Dijkstra

- [ ] **`src/Cardano/Benchmarking/Script/Core.hs:71-81`** - `withEra` dispatches on every era from Shelley to Dijkstra. Remove pre-Conway branches.
- [ ] **`src/Cardano/TxGenerator/Fund.hs:80-88`** - `getFundWitness` pattern-matches on every era in `InAnyCardanoEra`. Narrow to Conway and Dijkstra.
- [ ] **`src/Cardano/TxGenerator/Utils.hs:27-35`** - `liftAnyEra` pattern-matches on every era in `InAnyCardanoEra`. Narrow to Conway and Dijkstra.
- [ ] **`nix/nixos/tx-generator-service.nix:120-128`** - the `era` option enum lists shelley through conway. Narrow to conway and dijkstra.

### Selftest era migration

- [ ] **`src/Cardano/Benchmarking/Script/Selftest.hs:103`** - change `AnyCardanoEra AllegraEra` to `AnyCardanoEra ConwayEra`.

The era binding propagates consistently to all `AddFund`, `Submit`, and `createChange` actions - no witness mismatch.
The change is structurally safe: no golden files, no asserted transaction sizes, no hardcoded expected values exist.

However, the protocol parameter conversion takes a different code path for Conway (via `toConwayPParams` → `toBabbagePParams` → `toAlonzoCommonPParams`) and requires fields not needed for Allegra (`executionUnitPrices`, `maxTxExecutionUnits`, `utxoCostPerByte`, `costModels`, etc.).
The current `data/protocol-parameters.json` has `protocolVersion: {major: 6}` (Shelley/Allegra vintage) - semantically wrong for Conway.

- [ ] **Update selftest protocol parameters file** to `data/protocol-parameters-v9.json` (or v10), which already exists in the data directory with Conway-appropriate values (PlutusV1/V2/V3 cost models, `utxoCostPerByte: 538`, version 9).

- [ ] **`test/Bench.hs:21`** - uses `runSelftest` for criterion benchmarks. Only checks `Right`/`Left`, no size assertions. Verify it passes after the era + parameter file change.

Unaffected areas:
- SizedMetadata costs differ between eras (Allegra: 39 bytes first entry, Conway: 42 bytes) but are not exercised by the selftest (`TxMetadataNone` throughout).
- Plutus script handling is not exercised (selftest uses `PayToAddr` exclusively).
- Genesis configuration is not used (selftest loads protocol parameters directly from JSON).
- Transaction CBOR size will change (Conway body has additional fields) but is unasserted.

### Other files

- [ ] **`src/Cardano/TxGenerator/PureExample.hs`** - hardcodes `BabbageEra` throughout. Rewrite to Conway using the experimental API.
- [ ] **`src/Cardano/Benchmarking/GeneratorTx/SizedMetadata.hs:48-95`** - per-era cost properties from Shelley to Dijkstra. Keep for completeness.

## Definition of done

- [ ] tx-generator builds with no warnings on the experimental API.
- [ ] `selftest` CLI passes on Conway with the v9/v10 protocol parameters.
- [ ] `test/Bench.hs` criterion benchmark passes.
- [ ] No remaining sub-Conway transaction construction paths in `bench/tx-generator/`.
- [ ] Per-era dispatchers (`withEra`, `getFundWitness`, `liftAnyEra`) narrowed to Conway + Dijkstra.
- [ ] Nix era enum updated.