Feature: Add .explain_plan() method to QueryBuilder and SQL objects

[cofin]

Feature Request

Add an .explain_plan() method to QueryBuilder and SQL objects to help users understand query execution strategies, especially for complex MERGE and bulk operations.

Proposed API

# QueryBuilder usage
plan = (
    sql.merge()
       .into('products')
       .using(bulk_data)
       .on('t.id = src.id')
       .when_matched_then_update(name='src.name')
       .explain_plan()
)

# Output example:
{
    'strategy': 'json_populate_recordset',
    'row_count': 500,
    'param_count': 1,
    'estimated_cost': 'medium',
    'sql_preview': 'MERGE INTO products AS t USING (...) AS src...'
}

Use Cases

1. Debugging bulk operations - Understand which bulk strategy was selected
2. Performance tuning - See parameter counts and strategy choices
3. Learning - Help users understand how their query will execute
4. Documentation - Show examples in guides

Implementation Notes

• Should work on all QueryBuilder subclasses (Select, Insert, Update, Delete, Merge)
• Should show database-specific execution details
• Should not execute the query, only analyze it
• Optional verbose=True for detailed output

Related

• Deferred from merge-bulk-upsert implementation (Phase 3)
• Would complement .to_sql(show_parameters=True) method

Priority

Medium - Nice to have for developer experience, not critical for functionality