First-class Plasma and USD₮0 support for MPP EVM charge

[EfeDurmaz16]

Plasma is already usable through mppx's generic evm/charge path when the chain and asset metadata are supplied manually. Tether WDK also documents x402 payments on Plasma, so this is not a proposal to bring x402 to Plasma from scratch.

The remaining gap is first-class MPP ergonomics and coverage:

• Plasma mainnet uses eip155:9745.
• USD₮0 is 0xB8CE59FC3717ada4C02eaDF9682A9e934F625ebb, with 6 decimals and EIP-712 domain USDT0, version 1.
• mppx/evm has no Plasma preset, and mppx validate does not resolve Plasma USD₮0 as a known asset.
• viem/tokens.usdt0 is generated and currently does not contain Plasma, so the upstream metadata path needs agreement first.

I can take this as small PRs:

1. Resolve the preferred upstream source for Plasma USD₮0 metadata.
2. Add the agreed Plasma chain and asset ergonomics with focused runtime and public API tests.
3. Add validation, MPP/x402 signing coverage, and a short example.

Two questions before I start:

• Should the token metadata land upstream before mppx exposes a Plasma preset, or is a local known asset acceptable initially?
• Should the first pass be mainnet-only, given the lack of a clear public testnet USD₮0 and facilitator pair?