selfhost+test: wwstage type-info loss through struct-field N_INDEX (#37)

The original #37 symptom (worker-34's `..findflag` mangle) cannot
reproduce on master — was a runtime miscompile misattributed to a
link-time issue. Investigation surfaced three real wwstage cgen
gaps in the N_INDEX-through-struct-field family, sister bugs to
#34 (N_INDEX N_IDENT-base) and #36 (primsize-default-to-8).

1. `indexbaseesz` slice-element stride defaulted to 8 for named-
   struct elements. `&opts.ptr[i]` for `opts: *[]option` computed
   MOVQ $8 instead of $24. Fix: route slice case through
   `elemsizeofc(c, innert)`; ptr-to-named-struct via structlookup.

2. `cgun TK_AMP N_INDEX` ignored N_DOT base. `&p.ptr[i]` left
   esz=8 because only N_IDENT base was handled. Mirror cgindex's
   existing N_DOT arm.

3. `nodeisstr` N_INDEX arm only walked N_IDENT bases. `cmd.argsptr[i]`
   for `argsptr: *str` returned false; pushargsrev dropped the
   .len half at call sites. Add N_DOT-base arm that walks the
   struct field's pointee.

Test 713 (struct_field_index): 3 rows × 2 stages = 6 fixtures, one
per fix shape. Runtime-only; bootstrap byte-id (995_self_rebuild)
covers cross-stage drift.

Residual: getopttest's errortable still fails through wwstage with
a slice-of-str-via-&arr[expr] miscompile. Filed as task #38.

selfhost/cmd/wcc/cgenexpr.ww

@@ -2403,7 +2403,13 @@ fn cgun(c: *cgen, n: *node) void = {
 								};
 							};
 						};
-					};
+					} else { if (base.kind == nkind.N_DOT) {
+						// `&p.ptr[i]` shape: stride is the element
+						// of the slice/struct-pointer field, not
+						// the default 8. Mirrors cgindex's N_DOT
+						// arm so &p.ptr[i] and p.ptr[i] agree.
+						esz = indexbaseesz(c, base);
+					};};
 				};
 				cgexpr(c, idx);
 				if (esz > 1) {