ci: add coverage regression workflow

[rosterloh]

Summary

• Add a Coverage GitHub Actions workflow for the non-ROS crates with the quic feature enabled.
• Generate LCOV artifacts and step summaries for coverage runs.
• On pull requests, prepare current and base checkouts as non-ROS workspaces, generate base-branch coverage, and fail if current line coverage drops.
• Document local coverage usage and ignore generated coverage/ output.

Closes #18

Verification

• python3 .github/scripts/test_coverage_report.py
• python3 .github/scripts/test_prepare_non_ros_workspace.py
• CI-like local cargo test with .github/scripts/prepare_non_ros_workspace.py after temporarily moving ROS Cargo patches aside
• CI-like local cargo llvm-cov -p talos-common -p talos-cli -p talos-tui --features quic --lcov --output-path coverage/lcov.info via temporary local cargo-llvm-cov install; run outside sandbox so UDS tests could bind sockets
• python3 .github/scripts/coverage_report.py summarize coverage/lcov.info --markdown coverage/summary.md
• python3 .github/scripts/coverage_report.py compare coverage/lcov.info coverage/lcov.info --markdown coverage/comparison.md