feat: add support for RETURN DISTINCT keyword

[sonesuke]

Summary

• Add support for RETURN DISTINCT keyword
• Previously DISTINCT was not recognized as a keyword and treated as a column name, causing {"DISTINCT": "null"} in results

Changes

• Grammar: Add DISTINCT keyword, make it optional in return_clause
• AST: Add distinct: bool field to ReturnClause
• Parser: Detect DISTINCT token and set flag on parsed clause
• Executor: Deduplicate result rows when distinct is true
• Tests: Add parser and executor tests for DISTINCT behavior

Test plan

• cargo test — all 71 tests pass
• Parser correctly recognizes RETURN DISTINCT vs plain RETURN
• Executor deduplicates identical rows
• Pre-existing queries without DISTINCT remain unchanged

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