Add relative score scoreboard to benchmark report

[decofe]

Summary

• add a prominent higher-is-better fuzzer scoreboard for relscore and relcov
• support optional --relative-scores-csv input with approach/fuzzer, relscore/relscores, and relcov/covered_edges columns
• explain relscore as a coverage-value ordering score where rarer, consistently hit edges count more
• explain relcov as directional coverage overlap, with normalized fallback coverage used only when the report input lacks full pairwise relcov

Tests

• uv run --with-requirements analysis/requirements.txt python -m unittest analysis.tests.test_benchmark_report

Prompted by: @0xalpharush