Add initial Python SNS derivation utilities

[sitarua]

Summary

• Adds a minimal Python package scaffold for the in-progress Python SDK.
• Ports deterministic SNS derivation helpers for .sol domains, subdomains, reverse lookup accounts, and V1/V2 record accounts.
• Implements pure-Python base58 and Solana PDA helpers so the first slice has no runtime dependencies, network calls, wallets, signing, RPC, or transaction helpers.
• Adds unit tests using fixed fixtures generated from the existing JavaScript SDK.

Tests

$env:PYTHONPATH='python/src'
python -m unittest discover -s python/tests -v

Result: 7 tests passed.

Notes

• This is a small first slice, not a full Python SDK port.
• It intentionally avoids RPC helpers, wallet signing, private keys, transaction creation, and write operations.
• It does not claim assignment to any bounty or payment path.