Add test coverage for slashing mechanics (#20)

[Godbrand0]

Summary

Closes #20

• Adds a new contracts/trustflow Soroban contract crate implementing juror staking and a dispute-resolution system with stake slashing
• Jurors stake tokens before they can vote; minority voters on a resolved dispute are slashed by slash_bps / 10_000 of their current stake (default 10%)
• Writes 13 unit tests including the core acceptance criterion: three consecutive rounds of malicious voting produce a monotonically decreasing stake that converges to stake * 0.9^3 ≈ 7 290 (verified numerically)

Test plan

• test_stake_and_unstake – basic staking lifecycle
• test_stake_zero_rejected – invalid amount guard
• test_unstake_insufficient_funds – overdraw guard
• test_cast_vote_requires_stake – voting gate
• test_cast_vote_duplicate_rejected – double-vote guard
• test_single_round_malicious_juror_slashed – single-round 10% slash
• test_honest_jurors_not_slashed – majority voters unaffected
• test_multiple_rounds_malicious_juror_balance_decreases – core acceptance criterion: 3 rounds, stake decreases each round
• test_four_rounds_progressive_slashing – 4 rounds at 20% slash rate
• test_tie_favours_depositor – tie-breaking rule and minority slash
• test_resolve_dispute_no_votes_fails – edge case: no votes cast
• test_resolve_already_resolved_fails – double-resolve guard
• test_slash_cannot_exceed_stake – 100% slash rate floors at zero, not negative

All 13 tests pass. cargo fmt and cargo clippy --workspace --lib both clean.

[meshackyaro]

Good job and well done