[datlechin]

Summary

• View > Zoom In and Zoom Out (Cmd+=, Cmd+-) act on the area with focus, through the responder chain: a diagram zooms itself, a data grid changes the Data Grid font size, anything else changes the editor font size as before.
• DataGridScrollView answers both commands for every DataGridView: table and query results, Structure, Create Table, Compare, data file windows. It sits on the scroll view rather than the table because a cell being edited is mounted on the scroll view.
• The size is the Settings > Data value, so it persists and Settings shows it live. Each item dims at 10 or 24 pt instead of falling through to the editor.
• FontSizeZoom holds the one stepping rule the editor and grid share.
• Grid rows grow to fit a font taller than the Row height setting. Before, 22 pt clipped descenders at Normal and 18 pt at Compact.
• An open cell editor or viewer keeps its text and focus through a zoom and follows the new size. Before, the row-number column resize from any font change dismissed it without committing.
• The theme editor preview remounts when a font size changes elsewhere instead of keeping the old size.
• Titles stay Zoom In / Zoom Out. The command magnifies diagrams as well as resizing text, and Apple's apps put that under View > Zoom (Safari, Preview, Notes). The HIG's Bigger / Smaller belong to Format > Font, for styling selected text.
• No Actual Size: Cmd+0 is Show Sidebar, and a font has no actual size other than the one Settings holds.
• EditorZoomTests is now FontSizeZoomTests; its enablement case calls FontSizeZoom.editor.enablement in place of the removed MainSplitViewController.zoomEnablement.

Docs: TableProApp/docs#115

Test plan

• DataGridZoomCommandTests (hosted, real DataGridView): the chain from a focused grid and from a cell editor reaches DataGridScrollView before the window; zoom changes only the grid size; the items dim at both limits; rows grow with the font; an open editor with typed text survives a zoom that widens the row-number column and grows the rows. On main the grid has no responder, so the chain ends at the window.
• FontSizeZoomTests, DataGridRowHeightTests, and neighbours: DiagramZoomCommandTests, MenuValidationCoverageTests, CellOverlay*, DataGrid* suites.
• No screenshots: this session has no Screen Recording permission.

Refs #2526