add kani harnesses for solana programs

[kamiyo-ai]

Adds Kani formal verification harnesses for core Solana program crates (kamiyo, hive, kamiyo-staking), plus a small runner script.

• Harnesses live in programs/*/src/kani.rs behind cfg(kani)
• Adds Cargo check-cfg config for cfg(kani)
• Makes fee/reward split math overflow-safe (u128 intermediates) and avoids u128->u64 truncation in staking helpers

Run:

• cargo install --locked kani-verifier
• cargo kani setup
• ./scripts/kani.sh

Docs: KANI.md

[kamiyo-ai]

Superseded by #13 (clean Kani harnesses + CI integration).