feat: autoExpand option to grow the sheet on paste / autofill overflow

[righ]

Summary

Adds options.autoExpand — 'none' | 'vertical' | 'horizontal' | 'both' (default 'none', i.e. current behavior). When a paste or an autofill runs past the last row/column, the sheet appends the missing rows/columns instead of clipping the overflow.

<GridSheet initialCells={cells} options={{ autoExpand: 'both', limits: { maxRows: 1000 } }} />

• Paste: internal copy, cut→paste, and clipboard text/HTML (copy / copyAsync / move / writeRawCellMatrix).
• Autofill (Handsontable autoInsertRow-style, but provisional): while the fill drag's pointer is past the grid content's edge — in the whitespace of a larger box or outside it — and the grid is scrolled to that edge, provisional ghost rows/columns are added one at a time, faster the further past the edge (200ms → 20ms). They're display-only (no sheet change, no history); releasing grows the sheet up to the reached row/column, unreached ghosts vanish. Growth stops when the pointer is back over a cell.
• New rows/columns copy the last row's/column's layout (like inserting below the last one).
• Capped by limits.maxRows / maxCols; the rest is still clipped.
• One undo step: the growth and the write are recorded as a new BATCH history entry. If the operation is refused after growing (e.g. a move cutting through a merge), the growth is rolled back.
• A paste/autofill that grew the sheet scrolls the written range's far corner into view, so the growth is visible (the viewport keeps its size).

Implementation notes

• engine: Sheet.autoExpand, getAutoExpansion(), withAutoExpand() / withAutoExpandAsync(), history batching in _pushHistory; undo/redo switch extracted into _undoHistory / _redoHistory so BATCH can recurse. Autofill clamps the drag target to what the sheet can grow to.
• react-core (Tabular): ghost rows/cols are rendered as real table rows/cells (.gs-ghost-cell, .gs-ghost-th) so sticky headers and column alignment work; virtualize() keeps the trailing rows/cols rendered when scrolled past the content. While growing, the target follows the last ghost and the grid stays pinned to that edge; StoreObserver / ScrollHandle edge auto-scroll no longer snap the target back to the last real row/col there.
• Fix (pre-existing): releasing an autofill drag over a ScrollHandle submitted the fill twice (two undo steps). StoreObserver's capture-phase mouseup is now the only submitter, as its comment already intended.

Limitations

• No provisional ghost rows during autofill on sheets tall enough to hit the scroll-height cap (~600k+ rows); paste expansion still works there.
• Undoing a very large (>20k cells) expanding paste runs synchronously, without the progress bar.

Test plan

• Jest: packages/engine/src/auto-expand.spec.ts (directions, limits, BATCH undo/redo, writeRawCellMatrix, move, autofill, rollback on refused op) — engine suite 78/78
• e2e: e2e/auto-expand.spec.ts (paste grow + undo, none clips without scrolling, vertical only, grown cells scrolled into view; autofill: provisional rows held past the edge then committed in one undo, vanish when dragged back, keep growing when leaving an enlarged box before its whitespace fills (down/right), stop once the pointer is back over a cell)
• Full e2e: 107/107
• Storybook: Restriction/AutoExpand (mode switcher, resizable, centered, limits 1000 × 100)

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