Add ECS regression unit coverage

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Summary

• Add unit_ecs coverage for the newly always-built ECS module.
• Fix ECS_Count() to report live entities after destruction using the vendored EnTT entity storage live-list count.
• Document the new unit target in tests/README.md.

Risky behavior now covered

• ECS lifecycle before/after init and shutdown.
• Component lookup, setters/getters, bounded tag handling, iteration filters, and motion integration.
• Destroyed entities no longer remaining visible through ECS_Count().

Test files added/updated

• Added tests/unit/test_ecs.cpp.
• Updated CMakeLists.txt to register unit_ecs.
• Updated tests/README.md.

Why this reduces regression risk

Recent feature-flag cleanup made ECS part of the default client build path, increasing blast radius for client frame motion and Lua-facing gameplay helpers. These tests pin down the core ECS API contracts without requiring renderer, game data, or runtime UI state.

Validation

• cmake -S . -B build-coverage-tests -DBUILD_TESTING=ON -DBUILD_UNIT_TESTS=ON
• cmake --build build-coverage-tests --target unit_ecs
• ctest --test-dir build-coverage-tests -R unit_ecs --output-on-failure