feat: add partial vault release

[dangelo352]

Summary

• add remaining_amount tracking to vault state for tranche-based settlement
• add release_partial while preserving release_funds as a full remaining-balance release
• make redirect/cancel transfer only unreleased escrow after partial success payouts
• document partial release semantics and update the interface mapping

Tests

• cargo fmt --check
• cargo test --test partial_release -- --nocapture
• cargo test
• cargo clippy --all-targets --all-features -- -D warnings

Closes #218

[1nonlypiece]

appreciate the effort on this — but #218 isn't assigned to you, and we ask contributors to get assigned to an issue before raising a PR. closing as invalid; please claim #218 on the board first, then open it again and we'll take a proper look. https://discord.gg/xvNAvMJf