Field write extends the existing TY_TAGGED branch with an is_global
arm: LEAQ name(SB),CX after cgexpr (no AX/BX clobber), then MOVQ
into slot+foff+0 (tag) and slot+foff+8 (value, plus +16 for str-
typed variants).
Field read now treats tagged fields specially — load AX=tag,
DX=val0, CX=val1 (when union >16B), mirroring the tagged-return
ABI that let-init and match dispatch already expect. Previously
the scalar-load path read 8B into AX and left DX/CX with junk,
which silently broke local tagged-field reads too.
Hojun-Cho
328a53de5b