[baobao] Implement Solana chain module (closes #22)

[mkcash]

Closes #22

Summary

Implement Solana chain module featuring:

• SolanaClient for connecting to any Solana cluster (mainnet/devnet/testnet/local)
• SOL balance check
• SOL transfer
• SPL token transfer
• ATA creation utility
• Feature-gated behind solana feature flag
• Unit tests with deterministic keypair

Files Changed

• src/chains/solana.rs (new) - Solana client
• src/chains/mod.rs - solana module with #[cfg(feature = "solana")]
• Cargo.toml - optional solana deps
• examples/multi_chain.rs - Solana demo
• README.md - status update

Test

cargo test --features solana -p arka

Run

cargo run --example multi_chain --features solana

[abhicris]

Welcome to kcolbchain, @mkcash — glad you're here. 🌱

Here's what happens from this PR:

1. Our automated review looks for obvious issues (tests, secrets, size) within a couple of hours.
2. If it's clean and CI passes, we merge without back-and-forth.
3. If we need changes, we'll leave a specific comment — not a generic nit. Push another commit and we re-review.

While you wait:

• Run the repo's tests locally (see the repo README.md).
• Keep the PR scoped to one concern — bigger PRs land slower.
• Don't commit tokens or .env contents.

What happens after your first merge

• You're listed on https://kcolbchain.com/contributors/ (opt-out anytime via data/contributors-optout.txt).
• At 2+ merged PRs you're a Regular; at 5+ you're a Fellow — Fellows are the pool we invite first for paid research, partner-org gigs, and grant-funded work. Details: https://kcolbchain.com/docs/org-contribution-policy.html
• Looking for what to pick up next? https://kcolbchain.com/invitations/ lists open help-wanted / good-first-issue across every kcolbchain repo.

Thanks for writing the code. We're building this to last.

[abhicris]

🤖 Audit verdict: safe

Solana module implementation is benign: legitimate Solana SDK dependencies properly feature-gated, no hardcoded secrets in production code, clean RPC client abstraction with proper error handling, comprehensive tests, and good documentation.

Audited by the kcolbchain PR pipeline. See pipeline docs.