The session help color legend showed the four tier rows whenever
maturityEnabled was on, even with known-word highlighting off, where no
token is ever tinted. Extract isKnownWordMaturityLegendEnabled so the
legend requires both toggles, matching getKnownWordMaturityEnabled.
- Add `ankiConnect.knownWords.maturityEnabled`/`matureThresholdDays` config resolution with validation, warnings, and fallback to defaults
- Session help color legend shows one row per maturity tier (new/learning/young/mature) when maturity coloring is on, instead of a single known-words swatch
- Update docs and changelog entry to describe the new legend behavior