wcc/ww: chained-index str/slice element loads the full header

cgindex read one word for a str/slice ELEMENT of a chained index
(xs[i][j], f().s[i]) — the element-kind gate keyed on node shape and
missed non-simple bases, dropping the 24B/16B header load (review
finding #22). Key on the element-type stamp; 989_chainidx_run pins
cs==ww (red 3/8 pre-fix).

selfhost/cmd/w6c/main.combined.ww

@@ -24573,13 +24573,23 @@ fn cgindex(c: *cgen, n: *node) void = {
 			// stamped outer element tinfo (indexresult over the inner
 			// index's value type). cstage reads base->type->sub->size
 			// for esz (cmd/w6c/cgen.c:2070-2071). Drops the
-			// indexvaluetnode walk. Kept as its OWN arm (it does not set
-			// elemisstr/elemisslice) so the chained-index byte-id is
-			// preserved verbatim; the catch-all below would re-stamp it.
+			// indexvaluetnode walk.
+			// #22 (F7-c3): also stamp elemisstr/elemisslice off the same
+			// chased element tinfo. Without them a chained index whose
+			// element is a str/slice (`m[i][k]` over [N][M]str) loaded
+			// only the ptr word — the 24B/16B header (len/cap) was
+			// dropped (stale BX/CX) → garbage .len downstream. cstage's
+			// idx_eff path classifies the element uniformly via
+			// type_isstr/type_isslice (cmd/w6c/cgen.c); align ww UP by
+			// reading the SAME stamp the esz read above uses. CLASS-N:
+			// the corpus has no chained str/slice element, so the prior
+			// byte-id is preserved (this only fires on the missed shape).
 			let et: *tinfo = n.type_: *tinfo;
 			if (et != nil) {
 				esz = et.size: i32;
 				signed_elem = typeissigned(et);
+				elemisstr = typeisstr(et);
+				elemisslice = typeisslice(et);
 				float_elem = typeisfloat(et);
 				f32_elem = typeisf32(et);
 				elem_isarray = tinfoisarray(et);

selfhost/cmd/wcc/cgenexpr.ww

@@ -1875,13 +1875,23 @@ fn cgindex(c: *cgen, n: *node) void = {
 			// stamped outer element tinfo (indexresult over the inner
 			// index's value type). cstage reads base->type->sub->size
 			// for esz (cmd/w6c/cgen.c:2070-2071). Drops the
-			// indexvaluetnode walk. Kept as its OWN arm (it does not set
-			// elemisstr/elemisslice) so the chained-index byte-id is
-			// preserved verbatim; the catch-all below would re-stamp it.
+			// indexvaluetnode walk.
+			// #22 (F7-c3): also stamp elemisstr/elemisslice off the same
+			// chased element tinfo. Without them a chained index whose
+			// element is a str/slice (`m[i][k]` over [N][M]str) loaded
+			// only the ptr word — the 24B/16B header (len/cap) was
+			// dropped (stale BX/CX) → garbage .len downstream. cstage's
+			// idx_eff path classifies the element uniformly via
+			// type_isstr/type_isslice (cmd/w6c/cgen.c); align ww UP by
+			// reading the SAME stamp the esz read above uses. CLASS-N:
+			// the corpus has no chained str/slice element, so the prior
+			// byte-id is preserved (this only fires on the missed shape).
 			let et: *tinfo = n.type_: *tinfo;
 			if (et != nil) {
 				esz = et.size: i32;
 				signed_elem = typeissigned(et);
+				elemisstr = typeisstr(et);
+				elemisslice = typeisslice(et);
 				float_elem = typeisfloat(et);
 				f32_elem = typeisf32(et);
 				elem_isarray = tinfoisarray(et);

selfhost/cmd/wwdump/main.combined.ww

@@ -24573,13 +24573,23 @@ fn cgindex(c: *cgen, n: *node) void = {
 			// stamped outer element tinfo (indexresult over the inner
 			// index's value type). cstage reads base->type->sub->size
 			// for esz (cmd/w6c/cgen.c:2070-2071). Drops the
-			// indexvaluetnode walk. Kept as its OWN arm (it does not set
-			// elemisstr/elemisslice) so the chained-index byte-id is
-			// preserved verbatim; the catch-all below would re-stamp it.
+			// indexvaluetnode walk.
+			// #22 (F7-c3): also stamp elemisstr/elemisslice off the same
+			// chased element tinfo. Without them a chained index whose
+			// element is a str/slice (`m[i][k]` over [N][M]str) loaded
+			// only the ptr word — the 24B/16B header (len/cap) was
+			// dropped (stale BX/CX) → garbage .len downstream. cstage's
+			// idx_eff path classifies the element uniformly via
+			// type_isstr/type_isslice (cmd/w6c/cgen.c); align ww UP by
+			// reading the SAME stamp the esz read above uses. CLASS-N:
+			// the corpus has no chained str/slice element, so the prior
+			// byte-id is preserved (this only fires on the missed shape).
 			let et: *tinfo = n.type_: *tinfo;
 			if (et != nil) {
 				esz = et.size: i32;
 				signed_elem = typeissigned(et);
+				elemisstr = typeisstr(et);
+				elemisslice = typeisslice(et);
 				float_elem = typeisfloat(et);
 				f32_elem = typeisf32(et);
 				elem_isarray = tinfoisarray(et);