Add 100-token juror stake requirement

[vku2018]

Summary

• add a MIN_JUROR_STAKE threshold of 100 locked tokens for dispute voting eligibility
• expose get_min_juror_stake and is_juror_eligible helpers for callers/tests
• emit stake, unstake, and vote events around juror staking/voting state transitions
• add tests covering 99-token ineligibility, 100-token eligibility, and blocked voting below the threshold

Closes #22.

Validation

Run from contracts/:

• cargo fmt --all
• cargo test -p trustflow - 15 passed
• cargo clippy -p trustflow --all-targets - passed, with existing Soroban macro unexpected cfg(testutils) warnings under Rust 1.96
• cargo build -p trustflow --target wasm32-unknown-unknown --release - passed, with the same existing Soroban macro warnings