fix: flex container size should add gap size if gap is defined

[TtTRz]

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[Copilot]

Pull request overview

This PR fixes a bug in the flexbox layout algorithm where flex container sizes were not including gap sizes in their calculations. The fix ensures that when computing the final size of a flex container, both main-axis and cross-axis gaps between flex items are properly accounted for.

Key changes:

• Added total_gap field to FlexLine struct to cache the calculated gap for each line
• Updated container size calculations to include gap sizes for both main and cross axes
• Refactored code to avoid duplicate gap calculations

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
float-pigment-layout/src/algo/flex_box.rs	Adds total_gap field to FlexLine struct, calculates and caches gap values, and includes gaps in all container size computations (main axis, cross axis, and free space calculations)
float-pigment-forest/tests/custom/css_flexbox/gap.rs	Adds test case validating that flex items correctly shrink when gaps are present and container has a fixed size

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