feat: probe the mainnet proving service without deploying anything

The MetaMask path needs a mainnet prover and the sprint chat only ever offered
Sepolia URLs, so the assumption was that mainnet meant self-hosting. It does
not: both mainnet services are live and unauthenticated, and a registration
proof came back in 4.1s with 309,312 felts and PROOF1 facts.

The probe signs with Snip12CallSetSigner against an ordinary OZ account, which
means the pool's is_valid_signature fallback is a working second non-Ready
route into STRK20 that needs no custom account class. Proving is not what
blocks MetaMask on mainnet; the missing factory and account class are.

Dry run by default - proving is off-chain, so the question is answered before
anything is signed. --submit is a separate step.

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