tests: cover renderer regression map runner

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Summary

• Adds deterministic script coverage for scripts/renderer_regression_maps.sh using an isolated fixture and stub idtech3_server.
• Registers the new test_renderer_regression_maps CTest target and documents it in tests/README.md.

Risky behavior now covered

• GAME_BASE is translated into the expected fs_basepath parent and fs_game basename for all six renderer regression maps.
• Dedicated-server map-load args remain stable (dedicated, vm_game, bot_enable, com_hunkMegs, +map).
• Server-log error patterns and missing/invalid GAME_BASE fail the gate instead of silently passing.

Why this reduces regression risk

The map runner is the Tier B content-backed renderer validation entry point. Regressions in path derivation, map iteration, or failure detection could produce false confidence in renderer validation; this test catches those without requiring real game data or a GPU.

Validation

• tests/scripts/test_renderer_regression_maps.sh scripts/renderer_regression_maps.sh
• cmake -S . -B build-coverage-tests -DBUILD_TESTING=ON -DBUILD_UNIT_TESTS=OFF
• ctest --test-dir build-coverage-tests -R test_renderer_regression_maps --output-on-failure