feat(virtual): derive visibleCount and overscan from container size

[chiefcll]

Summary

Replaces VirtualRow / VirtualColumn with a version that derives window size from the rendered layout instead of asking the caller for it. The outer API is otherwise unchanged.

Removed props (all auto-derived now):

Removed	Replacement
displaySize	floor(container size / (first child size + gap))
bufferSize	max(2, ceil(visibleCount * 0.25))
uniformSize	always treated as uniform (was the default)
factorScale	dropped; layout uses unscaled item size

All other props (each, wrap, scrollIndex, onEndReached, onEndReachedThreshold, debugInfo, children, plus inherited scroll/selected/onSelectedChanged) are unchanged. VirtualGrid is untouched.

How it works

• On mount, a single probe item is rendered so the layout pass has something to measure.
• After the first updateLayout() the container's width/height and the first child's size are read; visibleCount, bufferSize, and itemSize are cached in a single signal.
• Once measurement completes, the full slice is rendered and the existing computeSlice state machine takes over — all the scroll / wrap / edge logic from before is preserved verbatim.
• Item size is assumed uniform for the lifetime of the component. Callers swapping to a differently-sized dataset should remount.

Caller-facing behavior change

• Callers must give the Virtual element a measurable width (Row) or height (Column) on first layout — either explicit, or sized by a flex parent. If the container has no size on the first measurement attempt, the effect re-runs when each updates.
• First paint shows one probe item; the full window appears on the next microtask.

Test plan

• npm run tsc — clean
• npm run lint — 0 errors (warnings are pre-existing)
• npm test — 120/120 passing
• Smoke-test a real app with VirtualRow and VirtualColumn to confirm one-frame measurement is imperceptible and scroll behavior matches v1
• Verify wrap, scrollIndex, and scroll: 'edge' | 'always' | 'auto' modes still behave as before

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