Port the Python calc layer to Rust: league stat distributions (avg excludes zeros,
stdev N-1 includes zeros), weighted standardized matchup scoring with switch-hitter
resolution and pitcher inversion, and SHA-256-validated score cache with automatic
rebuild after card imports. 105 tests passing (76 unit + 5 integration + 24 DB).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
