State correctness: add composed lifecycle transition-table invariant tests

[mark-e-deyoung]

Summary

Add explicit composed state-machine invariant tests across system lifecycle, API lifecycle, and UI lifecycle.

Why

Current tests validate many endpoints and idempotent paths, but there is no single transition-table test suite that codifies allowed/blocked transitions across composed states.

Scope

• Define transition table for:
  • Instance states (starting, running, shutting_down, powering_off, completed, failed)
  • Session states (active, suspended, completed)
  • API availability states (running, stopping, down)
  • UI polling states (normal, backoff, stopped)
• Add tests that validate expected outputs for key transition sequences (including repeated/idempotent calls).
• Add negative tests for forbidden transitions.

Acceptance Criteria

• Transition table is documented and referenced by tests.
• Tests fail on any undocumented transition behavior regression.
• CI runs the suite in pre-flight or deterministic integration stage.