feat(solana): support native SOL rewards for token locks

[arkoc]

Summary

• add atomic SPL-principal + native-SOL-reward lock instruction
• add matching redeem and refund instructions
• keep SOL reward as lamports in the solver-lock PDA and represent the native reward mint with the System Program address
• cover lock, redeem, and refund paths in the Anchor test suite

Safety

• principal and reward are escrowed in the same transaction
• redeem always sends SPL principal to the stored recipient and SOL reward according to the stored reward timelock
• refund always returns both assets to the stored refund address
• native-reward instructions reject non-SPL principal state and non-native reward state

Verification

• cargo check -p train-htlc
• cargo test -p train-htlc
• Anchor IDL generation tests: 10 passed
• ./node_modules/.bin/tsc --noEmit

The local Anchor validator suite was added but not executed here because Solana/Anchor CLI binaries are not installed in this environment.