fix: invalidate table scan schema cache

[fallintoplace]

Summary

• invalidate the cached scan snapshot schema whenever snapshot/ref/time-travel state changes
• preserve requested column-stat names and re-resolve them during Build so stats IDs match the final snapshot schema
• add regression coverage for IncludeColumnStats before UseSnapshot and non-main UseRef with an evolved schema

Tests

• cmake --build build --target scan_test
• ctest --test-dir build -R scan_test --output-on-failure