Add scrolling primitives: contentOffsetX/contentOffsetY props and client/scroll size metrics - #988

This implements the scrolling primitives sketched in #765, following the direction there: core primitives only, so a ScrollView can be built in userland (example included in the readme).

• contentOffsetX / contentOffsetY props on Box. Children are shifted left/up by the given number of columns/rows at render time. Combined with overflow="hidden", this is the scroll mechanism. Layout is untouched, the offset is applied in renderNodeToOutput when recursing into children, so Yoga sees nothing new.
• measureElement() now also returns clientWidth/clientHeight (size excluding borders) and scrollWidth/scrollHeight (content extent including overflow, computed from child layout extents and clamped to be at least the client size, matching DOM semantics). These extend the x/y/width/height shape from feat: extend measureElement() to return position coordinates #968.
• useBoxMetrics() returns the same four new fields, so a userland ScrollView can be fully reactive to content changes via the existing layout listener.
• The readme ScrollView recipe (and the runnable examples/scroll) wrap the content in a flexShrink={0} container. Without it, Yoga squeezes the children into the fixed-height viewport and there is nothing to scroll; the recipe notes this explicitly since it is the first thing anyone building a ScrollView will hit.

One deliberate divergence from the snippet in #765: child extents are measured relative to the inside of the container's border (parent border offsets are subtracted), so scrollWidth === clientWidth when content exactly fits a bordered box. The issue snippet measured against the outer edge, which over-reports the scroll extent by the border width.

Deliberately not included, per the discussion in #765: scrollbars, overflow indicators, an imperative scroll API, and any built-in ScrollView component. Offsets are also not clamped to the content extent, clamping policy belongs to the userland component (the readme example clamps via scrollHeight - clientHeight).

Tests: rendering tests for vertical/horizontal offsets, offsets inside borders, nested offset containers, offset beyond content, and a zero-offset behavior lock; metric tests for client/scroll sizes with overflowing content, borders, and content that grows after mount. All fail on master without the change (except the behavior lock). Verified manually in a real terminal with the included example (both axes, clamping at all edges).

Also relevant to #222.