feat: add Query Builder explain()

[memleakd]

Description

This PR adds explain() to Query Builder so applications can inspect the execution plan for the current SELECT query without manually compiling and prefixing SQL.

$query = $builder->where('status', 'pending')->explain();

The helper intentionally keeps the first slice small:

• MySQLi and Postgre use EXPLAIN.
• SQLite3 uses EXPLAIN QUERY PLAN.
• SQLSRV and OCI8 throw a clear DatabaseException because their explain flows need different mechanics.
• Test mode returns the compiled execution-plan SQL string.
• DBDebug=false query failures return false, matching normal builder execution behavior.
• The builder state resets by default, with explain(false) preserving it.

This also adds Model::explain() so model queries keep the expected soft-delete behavior, similar to other terminal builder operations.

Tests cover compiled SQL, SQLite syntax, reset behavior, failure behavior, unsupported drivers, live execution, and Model soft deletes.

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