fix: close wallet address and validation issues #295-#298

[emdy9008]

Summary

• Document and independently cover muxed ID 0 and u64::MAX; zero is valid, and the SQL allocator starts at 1 in signed BIGINT. Its transactional increment fails at the signed limit rather than wrapping, so the store cannot allocate u64::MAX.
• Add a corrupted-character regression for G... strkeys; PublicKey::from_string performs checksum-verifying decoding, and account validation intentionally accepts only base G... accounts.
• Version ownership challenges as v2 and bind the configured Stellar network passphrase into both the signed payload and HMAC. A challenge from one network is rejected on another; clients continue signing the returned opaque challenge string.
• Reject XLM and native credit-code spellings case-insensitively under Octo policy, with a distinct error. The added constructor cross-check asserts Asset::new_credit accepts those spellings, so this restriction is explicitly policy rather than a Stellar protocol constraint.

No compile or tests were run, as requested. git diff --check passed.

Closes #295
Closes #296
Closes #297
Closes #298

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