feat(xtask): add direct JSON compare mode

[mstykow]

Summary

• add a direct raw-JSON mode to xtask compare-outputs so saved ScanCode and Provenant outputs can be compared without rerunning either scanner
• infer info/classify/summary-style comparison sections from the provided JSON contents and cover the new mode with focused unit tests
• document the raw-JSON workflow in the xtask maintainer docs and update the CLI verification scorecard to reflect the new compare lane

Scope and exclusions

• Included:
  • xtask/src/bin/compare_outputs.rs
  • xtask/README.md
  • docs/implementation-plans/infrastructure/CLI_VERIFICATION_SCORECARD.md
• Explicit exclusions:
  • no public CLI/distribution changes yet; this keeps the capability in the maintainer harness for now
  • no scan-target packaging refactor beyond the new direct JSON mode

Follow-up work

• Created or intentionally deferred:
  • investigate exposing JSON-only compare publicly via the existing provenant CLI or a dedicated shipped binary
  • evaluate a longer-term conversion of standalone top-level modes into true subcommands instead of adding more flag-driven special cases