feat: Add ID-based model chunking

[memleakd]

Description

This PR adds chunkById() and chunkRowsById() to CodeIgniter\Model.

This builds on #9962, which added chunkRows() for handling whole chunks at once. The existing chunk() and chunkRows() methods use offset-based pagination. This works fine for read-only processing, but it may skip rows when records are updated or deleted while the chunk loop is running.

The new methods process records ordered by the model primary key instead:

$userModel->where('country', 'US')
    ->chunkById(100, static function ($user) {
        // ...
    });

For cases where the whole chunk should be handled at once:

$userModel->where('country', 'US')
    ->chunkRowsById(100, static function ($users) {
        // ...
    });

A few restrictions are added intentionally. Since these methods manage their own primary key cursor and ordering, they cannot be used with orderBy(), groupBy(), limit(), offset(), or union(). The model also needs a configured primary key, and the primary key must be selected when custom select clauses are used.

This should be useful for jobs, exports, cleanup commands, and other long-running processing where rows may change during iteration.

Tests cover normal processing, early exit, soft deletes, deleting rows while processing, invalid chunk sizes, missing primary key cases, and incompatible builder state.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide