codegen: fix struct-path TBAA for union array elements
When reading fields of a struct that was loaded from a union array
element, the data was written to a stack alloca via memcpy with
`jtbaa_arraybuf` TBAA. Subsequently, `best_field_tbaa` returned
struct-path TBAA tags (e.g., `{jtbaa_struct_UnitRange, jtbaa_Int64, 0}`)
for field reads from that alloca. These struct-path tags are in a
disjoint TBAA subtree from `jtbaa_arraybuf`, causing LLVM to conclude
the memcpy does not alias with the field loads — a miscompilation.
Fix by only applying struct-path field TBAA when the parent struct's
TBAA tag is compatible (the per-type tag, or generic value/data tags).
When the parent TBAA is an arraybuf or other leaf tag (indicating data
from a union array element), fall through to use the parent's TBAA
for field loads, preserving correct aliasing.
Also adds `type_metadata` alias scope region for GenericMemory metadata
fields, and properly propagates alias scope region for union selector
byte loads (previously defaulted to Region::unknown, losing all scope
information).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
