refactor(datagrid): unify cell edit path, class-based RowProvider cache

[datlechin]

Summary

• Single cell edit path: replace 5 duplicate recordCellChange + updateValue + reloadData sequences with a single commitCellEdit() method on TableViewCoordinator. ChangeManager is the source of truth for pending changes, RowProvider reflects display state, both updated atomically through one code path.
• Class-based RowProvider cache: replace @State dictionary cache (tabProviderCache) with RowProviderCache class. Cache writes are now synchronous (no more deferred Task blocks that could create duplicate providers during body evaluation).

Addresses DataGrid audit items #2 (dual state for cell edits) and #4 (RowProvider cache as @State).

Test plan

• Edit a cell via inline field editor, overlay editor, date picker, boolean dropdown, FK search popover, and "Set Value" context menu (NULL, Empty, Default). All should commit correctly.
• Undo/redo cell edits across all edit types
• Switch between tabs rapidly, verify no stale data displayed
• Edit cells, switch tabs, switch back, verify edits preserved
• Sort a table with pending edits, verify edits survive the sort

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[chatgpt-codex-connector]

Defer table reload until inline edit teardown completes

commitCellEdit now reloads the table synchronously, but control(_:textShouldEndEditing:) calls into this path while AppKit is still in the end-edit callback. Reloading the edited cell at that point can tear down the active field editor mid-cycle (e.g., Enter/Tab commits), which can lead to intermittent dropped commits or broken focus navigation. This path previously deferred reload to the next main-actor turn, and that deferral is still needed for inline text editing.

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