feat(sub-account-anonymizer): two-stage commitment + get_sub_accounts view (#883)

privacy_compute becomes hash(hash(identity_key, dapp_name), nonce); the inner
hash is the PartialCommitment. get_sub_accounts(partial, start, end) resolves each
nonce in range to a SubAccountInfo { nonce, address, is_deployed } — a deployed
nonce carries its stored address, an undeployed one the deterministic address it
would deploy to (via OZ calculate_contract_address_from_deploy_syscall, matching
the deploy syscall). Range capped by MAX_SCAN_RANGE.

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