[codex] fix ignoreMarginInRow item sizing

[nexiumbiz-debug]

Fixes #977.

What changed

• Centralizes the trailing item margin calculation for fixed-size axes.
• Stops subtracting a trailing row margin when ignoreMarginInRow makes the fixed row height the raw row size.
• Applies the same calculation to rendered item styles, cached item dimensions, and display-grid row/column styles.
• Adds regression coverage for the issue config: VerticalFixed, margin: 20, fixedRowHeight: 20, ignoreMarginInRow: true, and an item with rows: 6 now renders at 120px instead of 100px.

Root cause

updateGridster() already respects ignoreMarginInRow by not adding margin to the fixed row height for fixed and verticalFixed grids. Item sizing still subtracted margin unconditionally afterward, so 6 * 20px became 100px instead of 120px.

Validation

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