wcc+w6c_ww: aggregate-field stores via cgplaceaddr (C1.25)

Wire struct/array/tuple field STORE through the C1 assign-resolver
(task #23): structlit rhs materialises into a FRESH-per-use @placescr
slot (the @slicescr discipline — a cached slot is the #31 multi-live
trap) then word-copies to the resolved address; addressable rhs
(ident/global/dot/deref) sources via aggarg_srcaddr with the dest
spilled around the dispatch (#270-1b order). Kept loud: compound on
aggregate, sret call rhs (#234-tail), <=24B call rhs (task #24),
unaddressable literal rhs, ww-only anonymous-struct structinfo miss.

test/wcc/805: +6 rows (40B structlit incl ... autofill, ident+deref
source, nested literal, [3]u8 MOVW/MOVB and [3]u32 MOVL tails,
two same-size stores in one fn pinning fresh-per-use) +3 exact-text
reject rows. Tuple-field row blocked by the pre-existing tuple
param/let-init word-2 drops (tasks #32/#33, documented in-row).
p7b_capstore_only graduates byte-id and runs; p7_composed builds and
runs on cstage, wwstage stays behind the pre-existing #29 asserttyped
bail (verified identical at master with w6c_ww).

selfhost/cmd/wcc/cgenutil.ww

@@ -4283,7 +4283,8 @@ fn cgstructlitfill(c: *cgen, si: *structinfo, lit: *node,
 };
 
 // Thin wrapper preserving the BP-rel call shape used by cglet,
-// cgreturn, and cgassign N_IDENT-lhs N_STRUCTLIT.
+// cgreturn, cgassign N_IDENT-lhs N_STRUCTLIT, and the C1.25
+// @placescr materialise (cg_structlit_fill_bp's named twin).
 fn cgstructlitfillbp(c: *cgen, si: *structinfo, lit: *node, bpoff: i32) void = {
     if (si == nil) { return; };
     cgstructlitfill(c, si, lit, 0, 0, "", bpoff);