fix: implement deallocation for fixed-length lists with heap elements

[sumleo]

Summary

• The deallocate function in core ABI had a todo!() for FixedLengthList, panicking when flat deallocation was needed (e.g., list<own<resource>, 3>)
• The deallocate_indirect function was a silent no-op (=> {}), leaking memory for fixed-length lists containing heap-allocated elements like strings or nested lists
• For flat deallocation, reused flat_for_each_record_type with repeated element types to iterate and deallocate each element
• For indirect deallocation, reused deallocate_indirect_fields with a vec of repeated element types to correctly compute memory offsets

Test plan

• Added codegen test verifying cabi_post_ deallocation functions are generated for list<string, 3> exports
• needs_deallocate already correctly returns true for fixed-length lists with complex elements — the fix ensures the deallocation instructions are actually emitted
• All existing workspace tests pass

[alexcrichton]

Thanks! For testing this, could this be added to a tests/runtime/* test?

[sumleo]

Done. Extended the existing tests/runtime/fixed-length-lists/ test with:

• string-list-param, string-list-result, and string-list-roundtrip functions using list<string, N> in the WIT interface
• Custom allocator (alloc.rs) in both test and runner components
• allocated-bytes function to track heap allocation
• Guard pattern in the runner to verify no memory leaks after each call

This exercises both the deallocate (flat) and deallocate_indirect (pointer-based) code paths for fixed-length lists with heap-allocated elements.