Adds GTest unit tests for CDamageRing in src/helpers/DamageRing.hpp.
Covers:
- setSize: initial full damage, same-size no-op, resize re-damage, zero-size
- damage: in-bounds accept, empty/out-of-bounds reject, partial clip
- damageEntire: normal path and zero-size no-op
- rotate: clears current, preserves as previous, wraps beyond ring length
- getBufferDamage: age<=0 and age>ring return full screen, age=1 current only,
age=2 accumulates previous, full ring accumulation, coalescing fallback,
empty ring returns empty
- hasChanged: false initially, true after damage/damageEntire, false after
rotate, false for out-of-bounds damage
27 new test cases, all passing.
