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feat(virtual-scroll): sparse windowing over server-paged datasets (#295)

* chore: bump packageManager to pnpm@11.24.0

The pinned 11.22.0 no longer matched the installed pnpm, so pnpm tried to
self-manage the version and failed integrity verification against the
lockfile ("Cannot verify the identity of the @pnpm/exe.darwin-x64 native
binary"). That broke `pnpm run check`, and with it the pre-commit hook.

CI reads the version from this field (pnpm/action-setup@v6 is unpinned),
so no workflow changes are needed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* feat(virtual-scroll): sparse windowing over server-paged datasets

`addVirtualScroll` derived all geometry from the length of the data store,
so it could only window over rows already in memory. Passing `totalRows`
now opts into sparse mode: the plugin sizes the scroll container and
computes ranges against the full dataset while `data` holds only the
resident window.

- `totalRows` — dataset size, independent of what's loaded
- `dataOffset` — absolute index of `data[0]`
- `onRangeChange` — fires when the window moves, so the caller can fetch
  and evict; deferred to a microtask so handlers can update stores

Indices reported in sparse mode (`visibleRange`, `scrollToIndex`,
`virtualIndex`) are absolute. Rows in range but not resident simply don't
render, and the spacers absorb their height, so the container stays
correctly sized mid-fetch. Sparse geometry uses a uniform row height
(the running average of measured rows), since rows outside the window
can't be measured.

Dense mode is untouched — it stays the default and its behavior is
unchanged.

Adds a /virtual-scroll-sparse dev route that windows over a 4,000,000-row
paged source with LRU eviction, and documents the browser scroll-height
ceiling (~16.7M px in Chrome) that bounds addressable scrolling.

Closes #294

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* docs(virtual-scroll): drop unverified F... (continued)

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