Fix `repeat(auto-fill/auto-fit)` fill-count resolution

[FlorianRappl]

Description
Grid repeat(auto-fill/auto-fit, ...) does not compute true fill count correctly in all cases.

Scope

• Implement correct fill-count calculation from available space, track sizing constraints, and gaps.
• Distinguish behavior between auto-fill and auto-fit.
• Integrate with current grid track sizing algorithm stages.

Acceptance criteria

• Track count matches expected results across container widths.
• auto-fill vs auto-fit differences are observable in collapsed/empty tracks behavior.
• Tests cover minmax tracks, gaps, and edge widths.