feat(explain): include projection/order/limit stages

[aviralgarg05]

Summary

Enhance EXPLAIN output to include projection, ordering, and limit stages.

Why

Current explain output should make optimization/debugging easier by showing each logical stage explicitly.

Tasks

• Extend explain generation in Python bridge path
• Include fields for selected columns, ORDER BY clauses, and LIMIT value
• Add tests for formatted explain output

Acceptance Criteria

• EXPLAIN output includes projection/order/limit details
• Output remains readable in CLI
• Existing explain tests continue to pass

[coderabbitai]

🔗 Related PRs

#56 - Add EXPLAIN query plan command with AI optimization insights [merged]
#131 - feat: add EXPLAIN query plan command [merged]

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