cgen: default the hi bound of a slice/str-field slice, both stages

x.slicefield[:] / x.strfield[2:] emitted MOVQ $0 for the omitted hi
bound on BOTH stages (N_IDENT-gated dispatch; symmetric, so byte
identity never caught it) in all four sites: cgexpr N_SLICE + pushargs
(cstage), cgslice + pushargsrev (wwstage). The new arm re-evaluates
the pure field read for its {ptr,len,cap} header and takes .len,
covering local, viaptr, dot-chain, (*p), arr[i], and global inners.
Call inners still loud-reject upstream. Sibling of the #252/#257
array-field arms.

selfhost/cmd/wcc/cgenutil.ww

@@ -782,9 +782,16 @@ fn pushargsrev(c: *cgen, arg: *syntax.node, param: *syntax.node, memphase: bool,
 			emitline("\tMOVQ\t$");
 			emitint(dotbu.alen: i64);
 			emitline(", AX\n");
+		} else { if (dotbu != nil && (dotbu.kind == syntax.tykind.TY_SLICE
+		    || dotbu.kind == syntax.tykind.TY_STR)) {
+			// slice/str FIELD base — the arg twin of the cgslice
+			// default-hi arm; same re-eval-for-header rationale.
+			// Cstage twin: cgen.c pushargs N_SLICE.
+			cgexpr(c, base);
+			emitline("\tMOVQ\tBX, AX\n");
 		} else {
 			emitline("\tMOVQ\t$0, AX\n");
-		};};};};
+		};};};};};
 		emitline("\tPUSHQ\tAX\n");
 		// lo (default 0) → AX
 		if (lo != nil) { cgexpr(c, lo); }