cgen: resolve chained-DOT fn-ptr field callees

fnptrcalleetfn's N_DOT arm required an N_IDENT base, so a.b.cb(...)
fell to the name registry with an empty module hint and emitted
CALL cb(SB) (undefined symbol; cstage calls the stamped ptr indirect).
dotbasestructinfo resolves the base chain through the struct registry
— each link a struct- or *struct-typed field — and the single-dot
path routes through the same resolver unchanged. Closes the last open
shape of the #59.8 name-keyed callee family. Fixture fnptrfield_chain;
corpus pin 1487/2974.

test/wcc/data/fnptrfield_chain/case.ww

@@ -0,0 +1,27 @@
+//ww:run-exit 36
+package main;
+
+// A call through a chained-DOT fn-ptr field (a.b.cb(...)). The wwstage
+// callee resolver only accepted an N_IDENT base, so the chained shape
+// fell to the name registry with an empty module hint and emitted
+// CALL cb(SB) — the last open shape of the #59.8 name-keyed family.
+
+type bstr = struct {
+	cb: *fn(x: i32) i32,
+	pad: i32,
+};
+
+type astr = struct {
+	n: i32,
+	b: bstr,
+};
+
+fn twice(x: i32) i32 = { return x * 2; };
+
+fn main() i32 = {
+	let a: astr;
+	a.n = 1;
+	a.b.cb = &twice;
+	a.b.pad = 2;
+	return a.b.cb(20) + a.n + a.b.pad - 7;
+};