test: add unit tests for gapDemo module

**Difficulty:** Easy
**Expected outcome:** Add tests for `src/demo/gapDemo.ts` verifying:
- `runGapDemo()` returns correct keyword score (100/100)
- Execution score is < 20/100
- Gap is > 80 points
- All 20 test scenarios are present

**Files to create:** `tests/gapDemo.test.ts`