[codex] fix fixed grid size without outer margin

[nexiumbiz-debug]

Summary

• Fixes Gridster height does not exclude outerMargin when unused #920 by excluding the unused outer margin from setGridSize dimensions when outerMargin is false.
• Also covers Using VerticalFixed with setGridSize causes the grid to become wider on each update #845 by keeping verticalFixed + setGridSize from widening on repeated layout updates when the computed width feeds back into the next measurement.
• Keeps the existing extra outer margin when outerMargin is true.
• Adds component-level regression coverage for a 2x2 fixed grid with 100px cells and 10px margin sizing to 210px instead of 230px, plus a repeated-layout verticalFixed regression that stays at 300px.

Checks

• npm run test-lib -- --watch=false
• npm run build-lib
• npx eslint projects/angular-gridster2/src/lib/tests/gridster.spec.ts

Note: focused lint on gridster.ts still hits pre-existing unrelated upstream lint errors (component-selector, prefer-for-of, etc.), so I kept the lint check scoped to the touched regression spec.