[TESTS] Baseline engine test suite

[ibloud]

Summary

The engine needs a baseline headless test suite covering board geometry, aura application, Veiled state timing, and self-veil loss threshold.

Test Cases (minimum)

• Board geometry — valid and invalid coordinate checks on the 14x14 cross-grid
• Aura application — RadiusOfRuin correctly applies Veiled state to pieces within 1-square radius of Rebirth
• Veiled clearing timing — Veiled state clears at the start of the affected player's next turn, not immediately
• Veiled refresh — re-entering or remaining in aura refreshes duration rather than stacking
• Self-veil loss threshold — Rebirth loss condition triggers at correct self-veil count
• Immunity — Rebirth, Death, and Sanctuary-protected pieces are not Veiled by aura

Reference

• ARCHITECTURE_OVERVIEW.md — Veiled State Lifecycle
• ARCHITECTURE_OVERVIEW.md — Headless testability (Core Engine Principles)
• README.md — rulebook source of truth for all expected behaviors

Notes

All tests must run headless with no rendering dependencies. Snapshot comparison is the preferred assertion method once the serializer (Issue 4) is in place.