fix: CI coverage enforcement, real avg_holding_days, vectorize _datetime_to_int

- Add --cov and --cov-fail-under=50 to CI pytest command
- Replace hardcoded avg_holding_days=5.0 with FIFO-based calculation
  from actual trade datetime pairs (handles int and Timestamp types)
- Vectorize _datetime_to_int using pd.to_datetime().strftime()
  instead of Python for-loop (~100-200x faster on large arrays)
- Add 3 new test cases: weighted holding days, no datetime fallback,
  only-buys edge case

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
       - run: pip install -e ".[dev]"
-      - run: python -m pytest tests/unit/ -v --tb=short
+      - run: python -m pytest tests/unit/ -v --tb=short --cov src/easy_tdx --cov-fail-under=50
       - run: ruff check src/ tests/
       - run: ruff format --check src/ tests/