fix(lazy): handle empty item content without breaking mount termination

[chiefcll]

Summary

Alternative to #26, addressing the same bug plus the issues found while reviewing it. Thanks @girouxc for the diagnosis — the core insight (the mount decision must look at container.children.length, because items that render empty insert no child node) is adopted here.

The bug (#26): when items in a LazyRow/LazyColumn render empty, offset() overstates the rendered edge. selected (an index into actual children) then never gets within buffer of offset, so updateOffset early-returns forever and navigation stops mounting new items — the list dead-ends.

Why not exactly the #26 one-liner: replacing rendered with container.children.length in both halves of the guard introduces two new problems, because maxOffset (props.each.length) is in data units while children.length is in node units:

1. Multi-node item templates permanently stall mounting. Solid's universal renderer flattens fragments into direct siblings, so an item rendering 2 top-level nodes doubles children.length. With 20 items × 2 nodes, after 10 items mount children.length (20) >= maxOffset (20) — the guard early-returns forever and items 11–20 can never mount.
2. The termination guard goes dead in the empty-item case it fixes. children.length never reaches maxOffset when any item renders empty, so once everything is mounted the guard never short-circuits; with delay set, every keypress at the list edge clears and re-arms navDelayTimer around a no-op bump, indefinitely.

The fix: keep each comparison in its own unit system —

const rendered = container.children.length;
if (offset() >= maxOffset || selected < rendered - buffer()) return;

• Proximity (selected < rendered - buffer()) uses child units, matching selected — this is the [bugfix] Lazy components empty content handling #26 fix.
• Termination (offset() >= maxOffset) stays in data units, immune to the child count diverging in either direction.

Also in this PR

• Regression tests (tests/lazy.test.tsx) — there was no lazy coverage at all. One test reproduces the [bugfix] Lazy components empty content handling #26 scenario (items rendering empty; fails on main), one pins the multi-node-template case.
• VirtualGrid barrel-init fix — VirtualGrid called lngp.withScrolling(false) at module init through the package barrel, but index.ts re-exports withScrolling after VirtualGrid, so any import with the barrel as entry (including the new tests) threw withScrolling is not a function. It now imports the module directly.

Known remaining inconsistency (out of scope)

lazyScrollToIndex still mixes units: it sets offset = index + buffer (data units) but scrollToIndex(index) indexes children. With empty items the target child may not exist — downstream guards (children[index]?.setFocus(), withScrolling's bounds check) prevent a crash, but scroll/focus silently no-op. Fixing it requires mapping child indices back to data indices, which deserves its own change.

Testing

• npm test — 136/136 passing (11 files), including the two new lazy tests
• New empty-content test verified to fail against the pre-fix updateOffset
• npm run tsc and npm run lint clean

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