feat(chain): implement Solana chain module (#22)

[xiaofengzii]

Summary

Implements the Solana chain module for arka SDK as specified in issue #22.

Changes

• Chain enum: Added Solana variant with chain ID 101, RPC URL, native token (SOL), and explorer URL
• SolanaConnector: Full RPC client implementation with:
  • get_balance() - Get SOL balance
  • get_slot() - Get current slot
  • get_block_height() - Get block height
  • �ccount_exists() - Check if address exists
  • get_token_balance() - Get SPL token balance
  • �uild_transfer_ix() - Build SOL transfer instruction
  • �uild_token_transfer_ix() - Build SPL token transfer instruction
  • send_transaction() - Send signed transactions
  • �irdrop() - Airdrop SOL (devnet/testnet only)
• Token mints: Well-known Solana tokens (USDC, USDT, WSOL)
• Cluster constants: Mainnet, devnet, testnet, and local validator RPCs
• Tests: Unit tests for balance, slot, account existence, token mints

Acceptance Criteria Met

• New module chain/solana.rs implementing Solana RPC operations
• Support: connection, balance check, SOL transfer, SPL token transfer
• Uses solana-client crate
• Tests with mainnet RPC
• Updated examples/multi_chain.rs with Solana demo
• README updated

Testing

Tests connect to Solana mainnet RPC and verify:

• Balance queries work
• Slot queries return valid data
• Account existence checks function correctly
• Token mint addresses are valid

Note

Previous PR #28 was closed without merge. This implementation provides a cleaner, more complete solution with proper error handling and comprehensive test coverage.

Closes #22

[abhicris]

Welcome to kcolbchain, @xiaofengzii — 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.

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

[abhicris]

🤖 Audit verdict: safe

Legitimate Solana SDK integration with standard dependencies, correct token constants, public RPC endpoints, and no malicious payloads or credential leakage.

Audited by the kcolbchain PR pipeline. See pipeline docs.