feat: implement sort operation for arrays

[alvinreal]

Summary

Implements sort for ordering array elements as specified in #22.

New Operation

sort .field [asc|desc] [, .field2 [asc|desc]]

Sorts array elements by one or more keys with configurable direction.

Examples

sort .name asc
sort .name desc
sort .score desc, .name asc
sort .name

Behavior

• Default direction is ascending when omitted
• Null values always sort last regardless of direction
• Non-array values: no-op (returns value unchanged)
• Non-existent fields: treated as null (stable order preserved)
• Multi-key: primary key compared first, ties broken by secondary, etc.
• Supports integers, floats, and strings

Tests (11 integration tests)

• sort_name_asc — Alphabetical ascending
• sort_name_desc — Alphabetical descending
• sort_multi_key — Multi-key (score desc, name asc)
• sort_integers — Integer field sorting
• sort_floats — Float field sorting
• sort_strings — String field sorting
• sort_nulls_last_asc — Nulls last in ascending
• sort_nulls_last_desc — Nulls last in descending
• sort_nonexistent_field — Non-existent field → stable order
• sort_non_array_noop — Non-array → no-op
• sort_default_direction_asc — Default direction is ascending

Fixes #22