Resolve flexible lengths per CSS Flexbox spec section 9.7, on resolved Raxol.UI.Layout.FlexItems. Called from Flexbox.calculate_single_line_layout/5 for every flex line.

• free space computed from OUTER sizes (margins included; :auto margins count as 0 during sizing — positioning distributes into them)
• inflexible items and items whose clamped hypothetical size differs from their base size freeze BEFORE the loop
• iterative clamp-freeze-redistribute until no min/max violations (bounded by the item count)
• exact integer accounting: each round distributes whole cells via largest-remainder, so no free space is silently lost to div truncation

Not implemented (documented divergence): the fractional-flex-factor rule (sum of factors < 1 leaves proportional free space undistributed) — factors here are non-negative integers, so a fractional sum cannot occur.