test(Lazy): cover initial render, navigation loading, and reactive each

[chiefcll]

Stacked on top of #6.

Summary

Two changes:

1. tests/Lazy.test.tsx — 14 cases focused on the TV-app use case of a Column containing many LazyRows.
2. src/primitives/VirtualGrid.tsx — fixes a latent module-init bug that vitest exposed.

The VirtualGrid fix

VirtualGrid.tsx had this at module top level:

import * as lngp from '@solidtv/solid/primitives';
const columnScroll = lngp.withScrolling(false);

When the barrel is evaluated, exports populate in declaration order. VirtualGrid is exported before withScrolling in primitives/index.ts, so when VirtualGrid's module-init code runs, lngp.withScrolling is still undefined. Production bundlers reorder the deps and hide the cycle; vitest's vite-node SSR loader doesn't, so the barrel access returns undefined and throws on call.

Fix uses the pre-built scrollColumn constant already exported from ./utils/withScrolling.js — same direct-import pattern Row.tsx uses for scrollRow. As a bonus, deduplicates an identical withScrolling(false) call.

With this fix, the barrel loads cleanly under vitest, so Lazy.tsx does not need its own import workaround. The earlier draft of this PR refactored Lazy's imports; that's been reverted in favor of the real fix.

Test coverage

Initial render

• sync=true mounts upCount items synchronously
• !sync ramps up to upCount over time (verifies the 16ms setTimeout chain)
• Caps at each.length when shorter than upCount
• Renders nothing when each is null / false

Navigation-driven loading

• ArrowRight at the buffer edge mounts the next item
• Mounting stops at each.length
• No mount fires while selection stays inside the buffer window

Column of LazyRows (the real TV shape)

• Multiple LazyRows render their upCounts independently
• Loading more items in one row leaves siblings untouched
• ArrowDown / ArrowUp between rows preserves each row's loaded count

LazyColumn

• ArrowDown drives vertical loading

Reactive each

• Shrinking past selected keeps focus inside a valid index range
• Growing each.length does not auto-mount beyond the current offset

Cleanup

• Unmounting mid-preload-ramp produces no errors

Test plan

• npm run tsc — clean
• npm test — 134/134 (120 prior + 14 new)
• Barrel import smoke test: import * as lngp from '@solidtv/solid/primitives' now succeeds under vitest

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