Spec + plan + metadata + state for the ImGui Test Engine integration.
Enables the test engine via --enable-test-engine flag, bridges it through
the existing API hooks layer (4 new /api/test_engine/* endpoints + 4 new
ApiHookClient methods), and proves the full bridge with a smoke test.
The test engine enables high-fidelity simulation of docking, window focus,
panel visibility, drag-and-drop, and keyboard input that the current Hook
API cannot express. The API hooks remain the single communication boundary;
the test engine is integrated behind it.
This is Track 1 of a 3-track campaign:
Track 1: bridge + smoke test (this track)
Track 2: migrate docking/focus/panel tests
Track 3: visual regression via screenshot capture
Key risk: R1 (GIL-transfer crash) mitigated by Phase 1 Task 1.4 manual
verification checkpoint. Parallel-safe against the running tier2 taxonomy
branch and the enforcement_gap_closure track (zero file overlap).