wwstage: copy full tagged-element slot for N_DOT/N_INDEX-base index read (#261)

The #259 store fix unmasked a pre-existing latent cs!=ww in the tagged-
element READ via an N_DOT base (`x.o[i]`) / chained N_INDEX base
(`m[i][j]`): wwstage materialized the element as a SCALAR one-word load +
zeroed tag where cstage copies the full tagged slot — silently dropping
the tag/payload-high word (wrong variant). Three sites all keyed off the
same N_IDENT-only gate; cstage classifies TY_TAGGED for ANY base off the
checker-stamped element type. Align wwstage UP:

- cgindex (cgenexpr.ww): the N_DOT/N_INDEX-base arm now sets
  elem_tagged/elem_slot_sz from n.type_ (the stamped element tinfo),
  mirroring cstage cgen.c:8101 — the full-slot copy arms then fire.
- rhstaggedabicall (cgenutil.ww): the N_INDEX branch reads
  typeistagged(src.type_) for any base instead of an N_IDENT-only
  structural lookup, mirroring cstage's src->type keying — fixes the
  let-init / call-arg widen-source spill.
- forwardtagged (cgenstmt.ww): the return-path passthrough gate now
  accepts N_INDEX/N_DOT tagged rhs (which cgexpr materializes into the
  tagged ABI), not just N_CALL — fixes `return x.o[i]`.

read + call-arg + return + chained 2D all close by construction (one
materialization path). cstage unchanged (pure wwstage-align-up). 949
gains 9 #261 rows (i32 + explicit-void variant per shape proves the tag
survives) and flips the two #259 read-back rows to byteid=1.

selfhost/cmd/wcc/cgenexpr.ww

@@ -1272,6 +1272,25 @@ fn cgindex(c: *cgen, n: *node) void = {
 			elemisstr = isstrtype(c, etn);
 			elemisslice = isslicetype(c, etn);
 		};
+		// N_DOT / N_INDEX base (`x.o[i]`, chained `m[i][j]`): the
+		// element tinfo is n.type_ (the checker-stamped indexresult),
+		// same source the esz/str/slice/float flags read above. Mirror
+		// cstage cgen.c:8101 `esubu->kind == TY_TAGGED` — classified for
+		// ANY base, NOT gated on N_IDENT, and NOT excluding the nullable
+		// fold (slot_sz=8 degrades the copy arm to one MOVQ, matching
+		// cstage's fallback `MOVQ AX,BX; MOVQ (BX),AX`). #261: without
+		// this an N_DOT-base tagged element fell to the scalar
+		// loadopsz path and silently dropped the tag/payload-high word.
+		if (base.kind == nkind.N_DOT || base.kind == nkind.N_INDEX) {
+			let dt: *tinfo = n.type_: *tinfo;
+			if (dt != nil) {
+				if (typeistagged(dt)) {
+					elem_tagged = true;
+					elem_slot_sz = dt.size: i32;
+					esz = elem_slot_sz;
+				};
+			};
+		};
 	};
 	cgexpr(c, idx);
 	if (esz > 1) {