Add RotationVoted event for intermediate votes (#130)

[Iker2522]

Summary

vote_rotation only published events on terminal outcomes (execute or deadlock-cancel). An intermediate vote that neither passes nor deadlocks the proposal fell through silently, so an off-chain indexer watching only events couldn't see votes accumulate on an open proposal.

Added a RotationVoted event (same #[contractevent] pattern as RotationExecuted/RotationCancelled), published right before the Ok(None) return once both terminal checks are ruled out. No changes to vote-counting/majority/deadlock logic.

Test plan

• cargo fmt --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test pass locally
• CONTRACT.md updated if the public interface changed
• scripts/testnet-smoke.sh run against testnet, if this changes contract behavior in a way that affects the deployed flow
• What you manually verified: cargo test -p tholos — 70 passed, 0 failed, including the existing terminal-path tests (test_rotation_requires_majority_then_executes, test_deadlock_autocancels_rotation) unchanged. New test asserts the exact event via to_xdr() comparison.

Closes #130

[collinsezedike]

Two doc gaps in files this PR doesn't touch, so I can't anchor them inline. docs/src/CONTRACT.md's Events table and vote_rotation prose list RotationExecuted and RotationCancelled but not the new RotationVoted, so a reader concludes intermediate votes emit nothing, the exact gap this PR fixes on-chain. docs/src/ROTATION_DESIGN.md's indexer-facing event enumeration and its vote_rotation walkthrough have the same omission, so an indexer author following that doc still misses intermediate votes. Please update both, this PR's own checklist also left CONTRACT.md unchecked.