Add CPM fixtures for non-finish-start relationships

[jonathanmcmichael]

Background

Rustit's CPM engine implements finish-start, start-start, finish-finish, and start-finish relationship weights, but the initial tests exercise only finish-start networks. Scheduling practitioners and Rust contributors can make this behavior easier to trust by adding small, documented fixtures.

Suggested scope

Work in crates/rustit-schedule/src/lib.rs. Add deterministic unit tests for:

• start-start with a positive lag
• finish-finish with a positive lag
• start-finish with a clearly explained expected result
• at least one negative lag (lead) where the expected timing is unambiguous

Acceptance criteria

• each fixture names the activities, durations, relationship type, and lag
• assertions cover relevant early starts/finishes and project duration
• comments explain the expected scheduling behavior in working-hour units
• no calendar or date logic is introduced
• cargo fmt --all -- --check, clippy, and workspace tests pass

If a fixture reveals a calculation defect, document the failing case before expanding the implementation.