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feat(virtual-scroll): expose an unbuffered viewport range (#300)

* feat(virtual-scroll): expose an unbuffered viewport range

`visibleRange` is padded by `bufferSize` on both ends because it decides
what gets mounted, so it answers "what is rendered", not "what is seen".
Consumers wanting the latter — a footer tally, a scroll-progress readout,
a "jump to row" indicator — had to re-derive the sparse geometry in app
code against `totalHeight`, which is a copy that has to stay bit-for-bit
in sync with the plugin forever and silently desyncs when it doesn't.

Add `viewportRange` to `VirtualScrollState`. Sparse mode reads it off the
same `layout` anchor that positions the rendered block, so it cannot drift
from what is on screen, and the scroll-range compression applied above
`maxScrollHeight` is already undone. Dense mode gets a matching
`HeightManager.getViewportRange`; it is a second O(rows) walk, but
`derived` is lazy, so it costs nothing unless subscribed.

Notably this reports the final row at the true bottom of the dataset,
compressed or not — the off-by-one that a hand-rolled copy clamping
against `totalHeight` alone lands on.

Closes #298

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

* refactor(virtual-scroll): simplify the viewportRange plumbing

Cleanup pass over the previous commit, no behavior change.

- `getViewportRange` counts rows instead of indexing with a sentinel, so
  `start <= end` holds by construction rather than by a trailing
  `Math.min` repair. Verified equivalent to the previous form across
  18k fuzzed cases (variable and zero heights, negative and past-end
  scroll positions, zero-height viewports).
- Drop `SparseLayout.viewportStart`. It was an unconditional alias for
  `anchorIndex`, i.e. two names for one number with nothing keeping them
  in step; the one consumer now reads `anchorIndex` directly.
- Inline `trackedRange`, which had become a bare alias for
  `dedupedRange(source, notifyRangeChange)`.... (continued)

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