Add tuple rest element support

[yhk1038]

Summary

• Add tuple rest element support ([Type, *Type[]] syntax)
• Implement TupleRestElement IR node
• Validate rest element position (must be at end, only one allowed)
• RBS conversion fallback to union array (RBS doesn't support tuple rest)
• LSP: tuple type completions and hover support

Example

# Basic tuple (existing)
[String, Integer]

# Tuple with rest element (new)
[String, *Integer[]]    # String + rest of Integers
[Header, *Row[]]        # Header + rest of Rows

Test plan

• Parser unit tests
• IR unit tests
• E2E tests
• LSP completion tests
• All existing tests pass

Closes #44