Runtime object layout

Heap object header

• Word 0: type descriptor pointer.
• Optional: object size for debugging/profiling.

Type descriptor

• Size, alignment.
• Pointer bitmap/offset list for GC scanning.

String

• {ptr, len} where ptr points to UTF-8 bytes.

Array

• {ptr, len, cap} where ptr points to contiguous elements.
• Element layout comes from type descriptor.

Struct

• Heap-allocated with header + fields.
• Field offsets follow alignment rules.