feat: register a MetaMask-derived account in the mainnet STRK20 pool

scripts/mainnet-eth712-probe.mjs drives the path the Eth712 account class and
factory exist for: a local EVM key (cryptographically identical to what
MetaMask produces for eth_signTypedData_v4 and personal_sign - the on-chain
validator cannot tell them apart) signs its factory ownership message, deploys
its deterministic Starknet account, signs the pool's CallSet as EIP-712 typed
data, and submits its own InvokeV3. No Ready at any step.

0x7f5C9666...55da9 -> 0x36b11089...11ff94, deployed and registered on mainnet:
  deploy   0x4f550991adf6b5da36252e2fb097c871d74c146187862e171dee7b7dd16c02d
  register 0x78abd030f99f94b757aee36dd32cb77bc864ee6fc116503259a02e47b472617

First registration attempt reverted with "not deployed" - the prover proves
against head-10, and that block still predated the deploy. Retrying after more
blocks passed succeeded; same proving-block-depth constraint the Sepolia notes
already describe, met from the other side this time.

Both hashes go in strk20.json. README and the Sepolia doc are updated to keep
this precise: the mainnet contracts and one proven account exist, but the
browser app's Connect EVM wallet still only targets Sepolia - evm-strk20-account.ts
is not repointed yet, so this is confirmed by script, not by the shipped UI.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>