test: add proptest coverage to tholos-v2

[Christopherdominic]

Summary

Adds a proptest! suite to contracts/tholos-v2/src/test.rs, mirroring the structure and in-process (fork: false) configuration contracts/tholos/src/test.rs already uses for its proptest_vote_counting/proptest_initialize_bounds modules.

What's covered

A new proptest_settlement module generates a random number (0-6) of third-party voters with random weights (realistic range: min_resolution_bond..=500_000, well under max_position/max_total_weight), random sides, and random reveal participation, runs the dispute through to Resolved (whichever terminal cause the reveal pattern actually produces), then checks:

• prop_settlement_payouts_conserve_pool_and_match_formula — settlement pro-rata forfeiture splitting: settle()'s own return value matches the documented formula (amount + floor(amount * forfeited_pool / recipient_weight)) exactly for every position, no payout ever exceeds principal plus the entire forfeited pool, and the sum of every final credited balance (via get_credit) exactly equals eligible_total — payouts sum correctly and never exceed the pool, regardless of position weight distribution.
• prop_dust_credited_to_exactly_one_recipient — dust routing: the floor-division remainder from pro-rata splitting is credited to exactly one address (the deterministic dust recipient settle documents), never split across recipients, never dropped, and never paid to more than one address.

Anti-snipe deadline extension (the issue's optional secondary target) isn't included — the two required properties above turned out to need a fair amount of scaffolding on their own (see the module's doc comment for one non-obvious wrinkle: resolve_outcome can't lazily open the Reveal phase and then fail to fully close it in the same call without Soroban rolling back that phase transition too, so the harness has to guarantee at least one real reveal() call happens before it can force-close a round with unrevealed weight outstanding).

Testing

• cargo fmt --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo test --workspace --locked (100/100 passing in tholos-v2, including the 2 new proptest functions each running 96 generated cases)
• cargo build --workspace --lib --target wasm32v1-none --release --locked

Closes #106

[collinsezedike]

@Christopherdominic thank you for the contribution. This looks good, there is nothing to flag.
If you're looking for another issue to pick up, #72 is open and unassigned.
Merging now.