feat: cell merge (engine model + rendering, selection, context menu)

[righ]

Summary

Adds merged cells — the engine model plus rendering, selection, and context-menu operations in react-core.

Engine (@gridsheet/engine)

• A merge is stored on its top-left anchor cell as merge: { rows, cols } (new MergeSpan type); covered cells are derived. Works from buildInitialCells too ({ B2: { merge: { rows: 2, cols: 3 } } }).
• New Sheet API: merge({ area }), unmerge({ area }), getMerges(), getMergeAt(point), expandAreaByMerges(area).
• merge keeps only the anchor's value; covered values are discarded (undoable). A partially overlapped merge is absorbed.
• Structural ops keep merges coherent:
  • insert rows/cols strictly inside a merge grows it;
  • remove shrinks it (dissolves at 1×1); removing the anchor row/col hands the merge (and its layout) to the next surviving cell;
  • move carries a whole merge with its anchor (incl. cross-sheet);
  • sortRows refuses while any merge spans rows.
• Everything rides the existing history (undo/redo).

UI (@gridsheet/react-core, @gridsheet/web styles)

• Rendering: the host cell (the anchor, or the first rendered cell of the range when the anchor is scrolled out of the virtualized window) stretches its content over the whole range. Inner grid lines are painted with --gs-surface; the grid lines are the --gs-border backdrop showing through, so transparent would not hide them.
• Selection: pointing a covered cell chooses the anchor; arrows step over a merge from its far edge; Enter/Tab walk over covered cells; drag / Shift selections grow to include every merge they touch. The editor covers the whole range.
• Context menu: "Merge cells" / "Unmerge cells".

Copy / cut / paste

• Copy-paste carries source merges to the destination, clipped to it (and tiled when the paste repeats). Values-only paste keeps the destination's layout and drops values that would land on covered cells.
• A paste or cut-paste dissolves destination merges it cuts through (anchor outside the destination); a merge whose anchor is overwritten is replaced. A USER paste is refused when such a merge is Merge-protected.
• Cut refuses a source that cuts through a merge (the UI never produces one, since selections grow over merges). Redo of a move re-applies the dissolved merges.
• With no selection, copying a merge anchor copies the whole merge.

Prevention

• New operations.Merge flag (262144), included in ReadOnly (so ReadOnly's numeric value changes).
• A USER merge is refused when any cell of the merge-expanded area carries Merge, or when a Write-protected cell would lose its value. A USER unmerge is refused when the anchor carries Merge. SYSTEM (programmatic) calls are not restricted.
• Sheet.canMerge(area) / canUnmerge(area) decide whether the context-menu items are enabled.

Also fixed

• Editor debounced bare Shift/Alt keydowns (like any key), so the key they modify could be dropped when pressed within a frame — e.g. a quick Shift+Arrow right after a click selected nothing. Shift/Alt are now exempt, like Meta/Control.

Not in this PR

• autofill across merges;
• xlsx <mergeCells> import/export.

Test plan

• Jest: packages/engine/src/merge.spec.ts (40 cases: merge/unmerge, undo/redo, formulas, insert/remove, sort, move incl. cross-sheet, prevention, copy/paste, cut/paste).
• Playwright: e2e/merge.spec.ts (9 cases: rendering, pointing + arrows/Enter, Shift+arrow / drag / Tab, selection growth, context-menu merge/unmerge + undo/redo, scrolled-out anchor, Merge prevention, copy-paste, cut-paste) against Storybook Basic/Merge. The full e2e suite passed (99/99).
• typecheck:all, react-core typecheck.
• e2e/utils.ts: go() now honours STORYBOOK_URL, so the suite can run against a Storybook on a port other than 5233.

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