Add Tezos X Previewnet deployment

[skenaja]

Summary

Adds support for deploying WXTZ to TezosX Previewnet (chainId 128064),
an ephemeral (~1–2 month) test chain with no LayerZero infrastructure.

The contract is deployed unmodified by standing up a minimal EndpointV2Stub
that satisfies the single endpoint call (setDelegate) made from
OAppCore's constructor. Local WXTZ features (deposit/withdraw, ERC20,
permit) work; cross-chain send/lzReceive intentionally do not.

Changes

• hardhat.config.ts, chain-config.ts — register tezosXPreviewnet
  network (RPC, chainId 128064, Blockscout verifier) and add 128064 to
  testnetChains. The network has no eid because LayerZero has not
  assigned one for this chain.
• deploy/WXTZ.ts — map chainId 128064 to itself for the
  etherlinkChainId constructor arg, so deposit/withdraw work locally.
• contracts/mocks/tezosXPreviewnet/EndpointV2Stub.sol — new previewnet-only
  stub. Implements only setDelegate(address) with a stored mapping + event.
  Path makes its scope obvious; clearly marked as not-for-production in NatSpec.
• deploy/tezosXPreviewnet/EndpointV2Stub.ts — new deploy script that
  registers the stub under the deployment name EndpointV2, so the existing
  hre.deployments.get('EndpointV2') call in deploy/WXTZ.ts resolves to it.
  Guarded by a skip that no-ops on every other network.
• Lint/format — npm run lint:fix + prettier sweep across scripts,
  tests, configs, and docs (no behaviour changes).
• README.md — new Deployments section listing every mainnet/testnet
  address with explorer links, plus a sidenote documenting the previewnet
  stub workaround.

Deployments

Network	Chain ID	Contract	Address
TezosX Previewnet	128064	EndpointV2Stub	0xF01b3160F2926420C7416ddA62544969063269ED
TezosX Previewnet	128064	WXTZ	0xf4a2e3BA5C14f11c23c8c4351093C058D20542f2

Both contracts are verified on Blockscout.

Cleanup

The stub, its deploy script, and the previewnet network entries should be
removed when previewnet is decommissioned. All previewnet-only code is
contained under contracts/mocks/tezosXPreviewnet/ and
deploy/tezosXPreviewnet/ to make that easy.

Test plan

• npx hardhat compile succeeds
• npx hardhat test passes (no logic changes to WXTZ.sol)
• Confirm WXTZ on previewnet accepts deposit of native XTZ and returns it via withdraw
• Confirm the stub's setDelegate mapping reflects the WXTZ contract → deployer wiring set during construction
• Spot-check that npx hardhat deploy --tags EndpointV2Stub --network <other-network> is a no-op