Gherkin support

Scenarios, Backgrounds (merged per scenario, Rules supported), Scenario Outlines with Examples tables (expanded per row), and tags for --filter (one or many, OR-combined). Tags are echoed per scenario in .saffron/reports/latest.json and history.jsonl, so external tools can key results by a stable tag such as a test-case ID. And/But steps inherit their semantic kind (action vs assertion) from the preceding step.