Update NSDiffableDataSource to be more compliant with the macOS implementation

[gcasa]

There were some issues with the initial implementation. These changes attempt to address some of those issues.

AI Disclosure: AI was used in this change to help create tests, documentation, and detect bugs.

[Copilot]

🟡 Changes recommended

There are API/behavioral issues that should be corrected before merge (notably NSTableView’s nil-identifier registerNib:forIdentifier: handling, plus avoidable update-path reload overhead in the animated snapshot application flow).

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR refines GNUstep’s NSDiffableDataSource / snapshot behavior and expands API surface + tests to better align with expected macOS semantics, including item/section mutation operations, reload tracking, and additional provider hooks.

Changes:

• Extend NSDiffableDataSourceSnapshot with item move APIs, reload tracking, and stricter duplicate identifier validation.
• Add/extend diffable data source APIs (completion handler, supplementary/row/section-header providers, table row/section mapping helpers) and expand GUI tests accordingly.
• Harden NSTableView view-registration paths to avoid registering prototype views with nil identifiers.

File summaries

File	Description
Tests/gui/NSTableView/TestInfo	Adds/records NSTableView test suite metadata.
Tests/gui/NSTableView/columns.m	Adds coverage for registering prototype views with nil identifiers.
Tests/gui/NSDiffableDataSource/NSDiffableDataSource_snapshot.m	Adds tests for item moves and duplicate item identifier rejection.
Tests/gui/NSDiffableDataSource/NSDiffableDataSource_collectionView.m	Strengthens snapshot/data-source integration assertions; adds table diffable row-mapping coverage.
Source/NSTableView.m	Skips nil identifiers in prototype registration; changes registerNib:forIdentifier: nil handling.
Source/NSDiffableDataSource.m	Adds snapshot reload tracking + item moves; adds completion handler APIs; adds supplementary/row/header providers; improves table diff application behavior.
Headers/AppKit/NSDiffableDataSource.h	Exposes new snapshot ops and provider/completion APIs in public headers.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 2
• Review effort level: Lite

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