External sort for ORDER BY in streaming mode

[vmvarela]

Part of #69
Depends on #91 (disk-backed storage), #93 (streaming query validation)

Description

Implement external merge sort to support large ORDER BY operations in streaming mode without loading the full sorted dataset into memory.

Acceptance Criteria

• ORDER BY queries in --stream mode produce correctly sorted output for datasets larger than available memory
• External sort uses temp files (respects --memory-limit for merge buffer size)
• Temp files are cleaned up after execution (including on error/interrupt)
• Performance is reasonable: sorting 1GB CSV should complete in under 5 minutes on typical hardware
• Query validation (Streaming query validation: detect full-table-scan requirements #93) no longer blocks ORDER BY when external sort is available

Notes

• SQLite disk-backed temp store (Disk-backed large dataset support via SQLite temp storage #91) may already handle this via its own sort implementation — confirm before implementing custom sort
• If SQLite handles it, this issue may just be removing the validation restriction from Streaming query validation: detect full-table-scan requirements #93 and verifying correctness
• Only implement custom external sort if SQLite's built-in approach is insufficient