feat(flex): warn in dev when justifyContent requires an explicit width

[chiefcll]

What

Adds a dev-only console.warn when a flex container uses a justifyContent value that needs free space to distribute (center, flex-end, space-between, space-around, space-evenly) but has no explicit width set.

Why

A flex row container without a width shrinks to fit its children, leaving zero free space. All five of those justify modes position children from the free space (containerSize − totalItemSize − padding), so with a shrink-to-fit width they have nothing to distribute and silently collapse to flex-start.

It's actually worse than a no-op: when no width is set, elementNode seeds props.w = 0, and the shrink-to-fit container resize only happens in the flex-start branch of flexLayout.ts. The other justify branches compute against containerSize = 0, producing zero/negative offsets while the container stays width 0. These modes are effectively undefined without an explicit container width that exceeds the content.

Approach

This is an alternative to #38. Rather than silently auto-filling the parent width, it keeps the existing shrink-to-fit behavior unchanged and surfaces a diagnostic so the developer sets an explicit width themselves.

• Gated by isDev — no warning in production.
• Guarded by the existing flexFitsWidth condition, so it only fires when the container would actually shrink (skips flexBoundary: 'fixed' and column directions).
• Passes the node to console.warn so it's clickable to the offending element, matching other warnings in the file.

Notes for reviewers

• Behavior is unchanged; this is purely a dev diagnostic.
• npm run tsc passes clean.

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