Enhanced Test Coverage, GNU Radio Compatibility, and CI/CD Integration

[loic-fejoz]

This PR significantly strengthens the testing foundation of fsdr-cli, ensures seamless integration with GNU Radio Companion, and adds automated code coverage reporting to our CI/CD pipeline.

Key Changes:

1. Expanded Test Suite

• Parser Coverage: Added 14+ unit tests to tests/csdr.rs for previously uncovered csdr commands (e.g., fmdemod, timing_recovery, audio, and various load commands).
• End-to-End GRC Execution: Created tests/grc_run.rs to verify the behavioral correctness of flowgraphs derived from GRC definitions, including complex chains like FM demodulation and filtered AGC.
• GRC Fidelity (Roundtrip): Created tests/grc_roundtrip.rs to ensure that saving and reloading .grc files preserves 100% of the graph structure and metadata.

2. GNU Radio Compatibility

• Custom Block Definitions: Created a new gnuradio/ directory containing 11 custom YAML block definitions for fsdr-cli specific blocks (e.g., dsb, weaver_usb_cf, pattern_search). This allows generated .grc files to be opened and debugged directly in GNU Radio Companion.
• Metadata Support: Enhanced the internal GRC representation to support standard GNU Radio Options, ensuring generated files are strictly compatible with GRC 3.8/3.10.
• Documentation: Added a "GNU Radio Integration" section to README.md with instructions on configuring $GRC_BLOCKS_PATH.

3. CI/CD and Infrastructure

• Code Coverage: Integrated cargo-tarpaulin into the GitHub Actions workflow to generate coverage reports.
• Codecov Integration: Automated the upload of coverage data to Codecov and added status badges to the README.md.
• Code Cleanup: Performed a full cargo clippy review and refactored multiple instances of non-idiomatic vector initializations to use the vec![] macro.

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