[starter] Expand deterministic cancellation race regression coverage

[ashishki]

Problem

Cancellation is implemented for queued and running jobs, but terminal-state correctness benefits from an explicit table-driven interleaving matrix.

Bounded scope

Add deterministic tests for cancel-before-lease, cancel-during-run, completion-versus-cancel, timeout-versus-cancel, and repeated cancel requests using local PostgreSQL/Redis fixtures.

Acceptance criteria

• Each interleaving has an explicit expected terminal state and event sequence.
• Every case proves at most one terminal lifecycle event and zero duplicate-terminal invariant violations.
• Repeated cancel requests are idempotent or have a documented deterministic rejection.
• Tests avoid timing sleeps where controllable synchronization can be used.
• Existing behavior changes only if a failing regression test demonstrates an invariant defect.

Non-goals

No distributed consensus, exactly-once claim, new scheduler, or production load benchmark.