test: guard client demo record/playback invariants

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Summary

• Adds test_client_demo_regressions to guard the recent cl_demo.c extraction and client demo record/playback behavior.
• Registers the script in CTest and documents it in tests/README.md.

Risky behavior now covered

• Demo parser EOF/truncation/oversize checks before payload reads.
• Record/stop-record filename, overwrite, collision, and protocol-selection invariants.
• Demo playback protocol fallback/custom protocol handling, pure-server bypass/restore ordering, and state/compat setup before priming.
• Client demo module wiring and command add/remove lifecycle.

Test files added/updated

• Added tests/scripts/test_client_demo_regressions.sh
• Updated CMakeLists.txt
• Updated tests/README.md

Why this reduces regression risk

The recent demo-module extraction moved a core client flow with file parsing and protocol edge cases. This deterministic source-level test prevents future edits from weakening bounds checks, fallback behavior, or lifecycle wiring without requiring display, GPU, or game data.

Validation

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