docs: record the mainnet registration of a plain Starknet account

An OpenZeppelin account signing a SNIP-12 CallSet registered in the mainnet
STRK20 pool through the public proving service: no Ready, no custom account
class, 8.68 STRK all in. The hash goes in strk20.json - unlike the three
already listed, its sender is our own account running our own code path.

Also records what this does not prove: the passphrase-derived viewing key
cannot be rotated after registration, and a browser wallet signing the same
CallSet is untested because the digest is built outside typed-data encoding.

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