Compound `OP=` through an index (gs[i]/a[i]) or a bare ident (g) on a
tagged union silently misbehaved: cstage dropped the index compound and
plain-stored, and BOTH stages compiled an ident compound into an add on
the tag word -- byte-identical, so the gate stayed green while the tag
was corrupted. A compound op on a whole union is nonsense.
Gate the index plain-store arm on TK_ASSIGN so a compound falls to the
existing #133 reject (wwstage's byte-id twin); add a dedicated #21 ident
reject in both stages. This closes the compound half of the tagged-payload
write class (deref #18, dot #34 already reject).
#19 (global tagged-array static-init DATA) is a separate emitter, still open.
Hojun-Cho
33f940e17c