[codex] keep empty-cell drops inside grid bounds

[nexiumbiz-debug]

Summary

• clamp empty-cell drop items against the high grid bounds so default-sized items can still be dropped near the right/bottom edge
• keep negative/outside-before-grid positions rejected by the existing collision path
• add coverage for a 12-column grid where a 6-column default item is dropped at the trailing edge

Fixes #912.

Tests

• npm run test-lib -- --watch=false
• npm run build-lib