feat: implement where filtering for arrays and values

[alvinreal]

Summary

Implements where filtering as specified in #21.

New Operation

where <condition> — Filter array elements or gate non-array values

For arrays, keeps only elements where the condition evaluates to truthy. For non-array values, acts as a boolean gate (returns the value if condition is true, null otherwise).

Examples

where .age > 18
where .status == "active"
where .name != null
where .score >= 50 and .verified == true
where .role == "admin" or .role == "super"
where not .deleted

Tests (10 integration tests)

• where_age_greater_than — Numeric > comparison
• where_string_comparison — String == comparison
• where_null_check — != null filter
• where_compound_and_condition — and compound condition
• where_or_condition — or compound condition
• where_negation — not negation
• where_on_non_array_true — Boolean gate (passes)
• where_on_non_array_false — Boolean gate (blocks → null)
• where_empty_result — All filtered out → empty array
• where_all_pass — All pass → unchanged array

Fixes #21