tests: cover EDA event bus edge cases

[cursor]

Risky behavior now covered

• EDA event bus disabled-cvar gating, channel capacity, case-insensitive channel reuse, payload bounds, FIFO peek/pop/drain ordering, telemetry, and full ring-buffer wraparound.
• Fixes a discovered edge case where a full EDA ring buffer (s_qHead == s_qTail with s_qCount == EDA_MAX_QUEUE) was treated as empty by EDA_Pop/EDA_Peek.

Test files added/updated

• Added tests/unit/test_eda.c
• Added tests/stub_eda_runtime.c
• Registered unit_eda in CMakeLists.txt
• Updated tests/README.md

Why these tests reduce regression risk

• The EDA bus is shared game infrastructure used by AIML/Lua/game systems, so queue-ordering or drop bugs have a wide blast radius.
• The tests exercise actual C behavior with deterministic stubs and catch capacity/wraparound conditions that source guards would miss.

Validation

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