Add early payout requests

[SlaVKsVolks]

Summary

Implements early payout requests for #8.

• Adds early_payout_threshold_bps to group state.
• Adds admin set_early_payout_threshold configuration.
• Adds recipient-only request_early_payout.
• Transfers the currently contributed partial pot when the threshold is met.
• Advances the group to the next round after early payout.
• Adds a test covering threshold rejection, successful partial payout, and round advancement.

Verification

• cargo fmt --all
• cargo test
• cargo clippy --all-targets -- -D warnings
• cargo build

Closes #8