tests(cargo-kani): add solana agent escrow example

[kamiyo-ai]

Adds a self-contained cargo-kani example crate under tests/cargo-kani/solana-agent-escrow/ that models a few safety-critical rules common in Solana-style agent payment flows.

Included proof harnesses (all expected to verify successfully):

• Timelock authorization: only the agent can release funds before expiry; the API can release only after expiry.
• Value conservation for settlement splits: refund + payment == amount across dispute/inference/expired-escrow paths.
• Tiered oracle requirements: bounded in {3,4,5} and monotonic in value-at-risk.
• Escrow FSM: actions respect a simple transition table; terminal states do not transition.

The model is intentionally minimal (no Solana runtime / Anchor modeling) and has no external dependencies, to keep regression cost low while exercising Kani on agentic patterns (FSMs, threshold logic, conservation invariants).

To run locally:

cd tests/cargo-kani/solana-agent-escrow
cargo kani --harness timelock_policy_matches_release_rule
cargo kani --harness settlement_splits_conserve_value
cargo kani --harness required_oracle_count_is_monotonic_and_bounded
cargo kani --harness escrow_fsm_actions_respect_transition_table