w6c: sret-wire the chained-leaf call store (o.a.b = wide())

The chained value-struct walker's N_CALL arm handled only <=24B
register returns; an sret-class rhs fell to the scalar tail and
stored ONE word (the sret dest pointer) into the leaf -- silent
truncation in BOTH stages, byteid-blind. Point the callee's hidden
RDI at the BP-relative leaf slot (the single-dot #234 arm verbatim);
a ptr-root/global chain joins the #234-tail loud-stop family.

selfhost/cmd/wcc/cgenexpr.ww

@@ -12052,6 +12052,31 @@ fn cgassign(c: *cgen, n: *syntax.node) void = {
 					};
 					return;
 				};
+				// #234: over-cap sret STORE into a chained
+				// value-struct leaf — `o.a.b = wide();`
+				// (sretretsize > 0: a >24B struct OR an
+				// over-cap tuple). Point the callee's hidden
+				// RDI dest at the leaf slot so it writes the
+				// WHOLE value there; pre-#234 this fell to
+				// the scalar tail and stored ONE word (the
+				// sret dest pointer). c.sretdestoff is
+				// BP-relative ONLY, so a ptr-root/global
+				// chain needs the runtime RDI-pointer dest
+				// variant deferred to #234-tail and HARD-
+				// STOPS loud (rule 7).
+				if (n.rhs != nil
+				    && n.rhs.kind == syntax.nkind.N_CALL
+				    && sretretsizetn(c, leaftype) > 0) {
+					if (viacx) {
+						let m234: str = "#234-tail: over-cap tuple sret store to chained ptr/global dest unsupported\n";
+						os.write(2, m234.ptr, m234.len: u64);
+						os.exit(1);
+					};
+					c.sretdestoff = rootoff + totaloff;
+					cgexpr(c, n.rhs);
+					c.sretdestoff = 0;
+					return;
+				};
 				// TY_STRUCT terminal: three rhs shapes:
 				//   - N_IDENT: word-copy from the rhs local slot
 				//     (cgexpr is skipped — no whole-struct register