Report activity IDs involved in a cyclic CPM network

[jonathanmcmichael]

Outcome

When CPM rejects a cycle, return a deterministic set or path of involved ActivityId values instead of only CyclicNetwork.

Truth source

Use a synthetic network A → B → C → A plus an independent activity D. The diagnostic must include A, B, and C and must not include D.

Invariants

• acyclic CPM timings do not change
• IDs remain strongly typed
• diagnostic ordering is deterministic across runs
• the algorithm does not depend on activity names or insertion-order accidents

Expected scope

Refine the schedule error/diagnostic in rustit-schedule and add a Schedule Lab case.

Non-goals

Automatically breaking cycles, calendar logic, constraints, graphical schedule display, and vendor-specific error formats.

Acceptance and evidence

• the three-node fixture reports exactly its cycle members or a valid closed cycle path
• an independent node is excluded
• repeated calculation returns identical diagnostic order
• existing CPM fixtures remain unchanged
• cargo xtask verify passes

Dependencies: #10.