w6c+wwstage: len() of tuple-element slice reads .len not .ptr (#235)

len() special-cased only a plain N_IDENT slice operand (load .len at
BP+off+8) and an array operand (fold $alen); every other shape fell back
to a bare cgexpr(operand), which for a slice leaves AX=.ptr. A tuple-
element read (t.N) loads only AX=.ptr, so len(t.N) on a slice/str tuple
element returned the slice's .ptr word AS its length — a silent
miscompile, gate-blind because the bootstrap never does len() on a
slice-typed tuple element (sibling of the #234/#237 tuple-sret cluster).

Both stages: detect a slice/str tuple-element len() operand and load the
element's .len word directly at BP + element_off + 8, mirroring the
N_IDENT slice arm and the tuple-field-offset walk (element_off sums
preceding element sizes through the type table). Byte-identical asm
(rule 10). The separate tuple-element-read full-header gap is #238; a
leading-scalar mixed-tuple has its own pre-existing sret-layout cs/ww
divergence, filed apart from #235.

Test 903_tuple_elem_slice_len_run: 4 slice/str-only tuple rows (two/
three slices, str+slice, slice+str; distinct lengths), build+run both
drivers + cs==ww byte-id. 12/12 ok.

selfhost/cmd/wcc/cgenexpr.ww

@@ -3809,6 +3809,51 @@ fn cgcall(c: *cgen, n: *node) void = {
 								return;
 							};
 						};
+						// #235: len() of a tuple-element slice/str
+						// (`len(t.N)`). The tuple-element read leaves
+						// only AX=.ptr — no slice-header sibling (that
+						// gap is #238) — so the cgexpr fallback below
+						// returned .ptr AS the length. Load the element's
+						// .len word directly at BP + element_off + 8,
+						// mirroring the N_IDENT slice arm above and the
+						// tuple-field-offset walk (cgenexpr.ww N_TTUPLE).
+						if ((u.kind == tykind.TY_SLICE
+						  || u.kind == tykind.TY_STR)
+						  && a.kind == nkind.N_DOT
+						  && a.lhs != nil
+						  && a.lhs.kind == nkind.N_IDENT) {
+							let lc: *local = localfindnode(c, a.lhs.str);
+							if (lc != nil) {
+								let tn: *node = lc.tnode;
+								for (tn != nil && tn.kind == nkind.N_TNAME) {
+									tn = aliaslookup(c, tn.str);
+								};
+								if (tn != nil) {
+									if (tn.kind == nkind.N_TTUPLE) {
+										let idx: i32 = fldnumidx(a.str);
+										if (idx >= 0) {
+											let tp: *node = tn.list;
+											let foff: i32 = 0;
+											let i: i32 = 0;
+											for (i < idx) {
+												if (tp == nil) { i = idx; }
+												else {
+													foff += slotsize(c, tp.lhs);
+													tp = tp.next;
+													i += 1;
+												};
+											};
+											if (tp != nil) {
+												emitline("\tMOVQ\t");
+												emitoff((lc.off + foff + 8): i64);
+												emitline("(BP), AX\n");
+												return;
+											};
+										};
+									};
+								};
+							};
+						};
 						if (u.kind == tykind.TY_ARRAY) {
 							emitline("\tMOVQ\t$");
 							emitint(u.alen: i64);