fix: heap-allocate array structs in filter, map, split

[cs01]

Summary

• Replace alloca %Array / alloca %StringArray with GC_malloc(24) + bitcast in filter, map, and split codegen
• Same class of bug as the Set alloca fix (PR fix: replace silent 0.0 fallbacks with compile errors in member access #146) — stack-allocated structs become dangling pointers when stored in class fields
• Affected: generateNumericArrayFilter, generateStringArrayFilter, generateNumericArrayMap, generateStringArrayMapImpl, generateSplit (both empty and normal paths)
• Added regression tests: filter-map-in-class.ts and split-in-class.ts that store results in class fields

Test plan

• All 450 tests pass (2 new)
• Self-hosting (quick) passes
• CI