fix: retain restored feature statistics during database construction

[liuzhengyang699]

Constructing ProgramDatabase with a checkpoint db_path calls load(), which restores the MAP-Elites feature statistics. Later in the constructor, self.feature_stats = {} discards those restored ranges. The next candidate is consequently binned against a fresh scale: in the regression, a value of 25 in the saved 0–100 range moves from cell 2 to cell 5.

Initialize the empty statistics before loading the checkpoint so the restored values survive construction. Existing persistence tests explicitly call load() a second time, which masks this path; the new tests exercise construction alone.

Validation: 9 tests passed across tests/test_constructor_feature_stats.py and tests/test_feature_stats_persistence.py. Both new cases fail before the fix. Official isort/Black pre-commit hooks and git diff --check passed.

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