feat: support multi-milestone vaults

[dangelo352]

Summary

• replace the single milestone hash/boolean with ordered milestone vectors and per-index validation flags
• update validate_milestone to validate a specific index with duplicate/out-of-range guards
• require all milestones for pre-deadline release while preserving deadline-based release and redirect behavior
• document the multi-milestone model and update the interface mapping

Tests

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

Closes #220

[1nonlypiece]

hey, good work, but this can't go in until you're assigned to #220 — that's our process to avoid two people on the same thing. closing for now; claim the issue first and re-raise. https://discord.gg/xvNAvMJf