Streaming query validation: detect full-table-scan requirements

[vmvarela]

Part of #69
Depends on #92 (chunked CSV reading)

Description

In streaming mode, some SQL operations require seeing the full dataset before producing output (e.g. unbounded ORDER BY, certain JOINs). Detect these cases and emit a clear error or warning instead of silently producing wrong or truncated results.

Acceptance Criteria

• Queries with unbounded ORDER BY in --stream mode emit a clear error message
• Queries with multi-table JOINs that aren't supported in streaming mode emit a clear error
• Error message explains the limitation and suggests removing --stream or rewriting the query
• Safe queries (WHERE, LIMIT, bounded GROUP BY) pass validation without warnings
• Validation runs before any data is processed (fail fast)

Notes

• Parse the query AST or use SQLite's EXPLAIN to detect full-scan operations
• Keep the validator conservative: false positives (blocking a safe query) are better than false negatives (silently wrong output)