layout: add weighted child sizing

[GGRei]

Summary

Add gui.weighted() for assigning proportional main-axis space to container children.

A row distributes width, while a column or circle distributes height:

gui.weighted(weight: 2, view: gui.button(...))

The implementation:

• supports constrained 1:1:2-style layouts;
• redistributes space correctly around minimum and maximum sizes;
• recalculates proportions when the parent is resized;
• preserves the existing layout path when no weights are used;
• adds no extra Layout node and does not change the View interface;
• rejects invalid weights and unsupported configurations explicitly;
• includes documentation and a resizable button example.

Validation

• Weighted layout tests pass with Clang in normal and production modes.
• Existing layout, wrap, border, GC leak and GC lint tests pass.
• Documentation and formatting checks pass.
• No allocations occur during distribution after warm-up.
• examples/weighted_layout.v compiles cleanly with -W and runs successfully.

Fix #71
Codex review validation

[squidink7]

Works well in my tests! My only question is to whether view as a property name is consistent with the rest of this library which seems to use content, however that usually refers to several child views while this takes only one. Is there anywhere else where this is the case?

[GGRei]

Thanks for checking this. The closest existing precedent is OverflowItem.view View, which also holds one arbitrary view.

There are singular content cases such as ExpandPanelCfg.content, but there content specifically represents the panel body. weighted is a transparent decorator, it applies metadata to exactly one view and adds no container or layout node.

For that reason, I think view is the more precise name here.