refactor(datagrid): native focus ring, single-click edit, a11y fixes

[datlechin]

Summary

DataGrid audit Phase 2 — native cell rendering and interaction improvements.

#	Issue	Resolution
#6	Single-click edit	Click an already-focused cell starts the field editor. Guarded against drag-select.
#8	System focus ring	drawFocusRingMask() + NSColor.keyboardFocusIndicatorColor instead of CALayer + #007AFF.
#9	VoiceOver above 5k rows	Drop the isLargeDataset accessibility gate.
#50	Hardcoded font 13 in date picker	Pull pointSize from ThemeEngine.shared.dataGridFonts.regular.
#51	Border color	keyboardFocusIndicatorColor instead of selectedControlColor.
#52	a11y row/col indices	setAccessibilityRowIndexRange / setAccessibilityColumnIndexRange per cell.

Audit items #19, #20, #54: NOT FOUND in current code; skipped.

This is the second of three planned PRs from the audit. Phase 1 landed in #919; Phase 3 (interaction fixes: NSUndoManager, multi-cell paste, etc.) and Phase 4 (cleanup) follow.

Test plan

• System Settings > Accessibility > Increase contrast: focus ring follows
• System Settings > Appearance > Accent color (e.g. Graphite): focus ring follows
• Click an unfocused cell: row selects, no edit
• Click the same focused cell again: field editor opens
• Drag-select starting from a focused cell: no edit triggered
• Date column: cell click opens popover; picker font matches data-grid font setting
• Multiline overlay editor: 2px border uses keyboardFocusIndicatorColor
• VoiceOver on a 10k row table: every cell announces correctly with row/column indices
• No regression on FK / JSON / blob / enum / set / boolean columns: their popovers still open via chevron / double-click
• Right-click > Set Value on a NOT NULL column: no "NULL" option shown
• Right-click > Set Value on a column without a default: no "Default" option shown

Files

• DataGridCellView.swift — native focus ring via drawFocusRingMask, restored final
• DataGridCellFactory.swift — drop CALayer border, drop a11y gate, add row/col index ranges
• CellOverlayEditor.swift — keyboardFocusIndicatorColor
• DatePickerCellEditor.swift — themed font size
• KeyHandlingTableView.swift — single-click edit on focused cell, drag guard
• Extensions/DataGridView+Editing.swift — InlineEditEligibility enum, shared inlineEditEligibility predicate
• Extensions/DataGridView+Columns.swift — restore isBooleanType in hasSpecialEditor
• TableRowViewWithMenu.swift — gate Set NULL / Set Default by column metadata

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: ec3cc8c396

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".