Extended cg_widen_tagged_store (cstage) / cgwidentaggedstore (wwstage)
to take a base_reg/basereg parameter so the primitive supports non-BP
destinations. Cstage extends body in-place via via_outer gate +
spill+scratch+copy-out; wwstage splits into wrapper (non-BP) +
cgwidentaggedstorebp (BP-only) to dodge the no-goto constraint. New
N_ASSIGN field TY_TAGGED branch routes through the primitive for all
rhs shapes.
Scope-adjacent: fieldsize recurses through N_TTAGGED via slotsize and
TNAME-aliased-to-tagged via aliaslookup. Needed for the test fixtures.
Wwstage read-side N_DOT-of-tagged-field source is filed as task #28;
test rows use mark-canary verification until that lands.
Parallel to TY_STR/TY_SLICE branches at cgen.c:1939/1962. Word-copy
from src slot to field+k*8 via AX, MOVL/MOVB ragged tail. N_IDENT
rhs only — struct-call-result and struct-literal rhs are different
code paths, filed as task #27. Symmetric in N_ASSIGN field case AND
spine-walker terminal; mirrored in wwstage cgassign four sub-shapes
(*struct base, direct local, global, spine terminal).
Parallel to the existing TY_STR branch at cgen.c:1939. Stores AX/BX/CX
at field+0/+8/+16 across three sub-shapes (via_ptr, is_global, direct
local) — DX as addr scratch where needed so CX (cap) survives.
Symmetric in wwstage cgassign. Surfaces tasks #25 (whole-struct rhs)
and #26 (whole-tagged rhs) in the same locus class.
Symmetric write-side counterpart of #8, bundled across both stages.
cstage [N]Struct write was broken (1986 gated on TY_PTR); wwstage had
no N_DOT(N_INDEX) write branch at all. New branch covers both
[N]*Struct and [N]Struct via viaptr flag, uses fldstoreop for
scalar/sub-word, MOVSS/MOVSD for float, two-MOVQ for str rhs.
Compound (PLUSEQ etc.) wired for integer scalar.
