List each strict violation once in JSON and CSV output

[dduugg]

Summary

pks check -o json and -o csv emitted every unrecorded strict violation twice, as byte-identical entries. On contains_strict_violations, main prints two identical CSV rows, and the JSON has two identical violations with violation_count: 2 next to strict_violation_count: 1.

The cause is that an unrecorded strict violation is in both reportable_violations and strict_mode_violations, and both formatters built their list with chain! over the two sets. They now take the sets' union, so each violation appears once and violation_count counts distinct violations. That also makes the schema's description of strict_violation_count as a subset of violation_count true.

Text output and CheckAllResult are deliberately unchanged. Text lists the violation in the report and again as a strict-mode message, which matches packwerk: OffenseCollection#add_offense puts every unlisted offense in new_violations whether or not it's strict, and CheckCommand shows both outstanding_offenses and unlisted_strict_mode_violations. Only the two formatters merged the sets into one list.

This is one of the two follow-ups from the #43 review. The other is #64, which moves strict onto Violation. The two branches merge cleanly in either order.

Tests

• test_check_with_strict_mode_output_csv now compares whole lines. With contains, it passed while the duplicate row was there.
• New: JSON on contains_strict_violations, plus CSV and JSON on uses_strict_mode with --ignore-recorded-violations, where recorded strict violations are in both sets.
• All four fail against a main build: two identical rows, violation_count 2 instead of 1, four rows instead of 2, and violation_count 4 instead of 2.

Test plan

• cargo test, 269 passed, plus fmt and clippy.
• Compared against main on simple_app, the strict fixtures, contains_package_todo and contains_stale_violations, with and without --ignore-recorded-violations. Text output is identical. JSON and CSV contain the same distinct entries, now with no duplicates.
• CI passes.