Add partial milestone release support

[alanamind7]

Closes #218

Summary

• Add partial milestone release support so an authorized verifier can release only part of a funded vault amount.
• Track remaining vault balance and keep the vault active until the final release completes it.
• Preserve existing access control and terminal-state protections for cancelled, failed, and completed vaults.
• Document the partial release flow and add focused coverage for partial release lifecycle behavior.

Validation

• git diff --check
• cargo test attempted on the VPS, but the installed Cargo is 1.75.0 while this repository declares rust-version 1.80 and uses Cargo.lock v4, so the VPS toolchain cannot parse the lockfile. The PR is ready for CI or a local Rust >=1.80 test run.

Notes

• The change is scoped to the milestone release path and avoids custody/transfer behavior outside the contract's existing authorization model.