feat: add closure-based Query Builder join conditions

[memleakd]

Description

This PR proposes a closure-based way to build Query Builder JOIN ON clauses.

Today, simple joins are easy:

$builder->join('orders', 'orders.user_id = users.id');

But once a join needs bound values, OR conditions, or grouped ON (...) logic, users often have to fall back to longer raw SQL strings or RawSql. That works, but it gives up some of the safety and readability the Query Builder usually provides.

This PR keeps the existing join() API intact and adds a native option for those more complex join conditions:

use CodeIgniter\Database\JoinClause;

$builder->join('orders', static function (JoinClause $join): void {
    $join->on('orders.user_id', 'users.id')
        ->where('orders.status', 'paid');
});

For grouped conditions:

$builder->join('orders', static function (JoinClause $join): void {
    $join->on('orders.user_id', 'users.id')
        ->groupStart()
            ->where('orders.status', 'paid')
            ->orWhere('orders.status', 'pending')
        ->groupEnd();
});

This produces a protected, bound JOIN ON clause without requiring users to manually write the whole condition as raw SQL.

The closure receives a JoinClause object with familiar Query Builder-style methods:

• on() / orOn() for column comparisons
• where() / orWhere() for bound value comparisons
• groupStart() / orGroupStart() / notGroupStart() / orNotGroupStart() / groupEnd() for grouped conditions

Existing string and RawSql join conditions continue to work as before. RawSql remains the explicit escape hatch for cases that do not fit this small join-condition API.

Why

This helps real-world applications express safer and clearer joins, especially when joins include conditional matching, status filters, soft-delete checks, tenant constraints, or grouped OR logic inside the ON clause.

Instead of choosing between a simple protected join string and a fully raw condition, users get a CodeIgniter-native middle path that keeps identifiers protected and values bound.

Changes

• Added CodeIgniter\Database\JoinClause.
• Added closure support to BaseBuilder::join().
• Reused the shared join condition compiler from the SQLSRV builder.
• Updated Model PHPDoc for forwarded join() calls.
• Added user guide docs and changelog entry.
• Added focused tests for escaping, binds, grouped/nested conditions, invalid group state, aliases, and SQLSRV table naming.

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