cgen: key the str/slice arg recognizers off the checker stamp

The wwstage nodeisstr/nodeisslice recognizers were name-keyed for
every non-local shape: an indirect fn-pointer callee ((*f)() — the
errnotest #59.5 divergence, streq receiving a shifted register file)
and module-global let/const idents (path.sepstr — union-widen pushes
zero-filled len/cap) both fell to false while cstage keys off the
checker stamp unconditionally. Both recognizers now fall back to the
stamped n.type_; push and pop sites share them, so the drain stays
balanced by construction. Graduates the #59.5 errnotest pin.

test/wcc/data/globalconst_str_widen/case.ww

@@ -0,0 +1,24 @@
+//ww:run
+// A module-global const str widened into a tagged-union call arg.
+// The pre-fix wwstage nodeisstr N_IDENT arm knew only frame slots and
+// defs, so the widen push took the scalar arm and zero-filled len/cap
+// (path.sepstr's strings.hasprefix shape).
+package main;
+
+const gs: str = "hello";
+
+fn f(x: (str | i32)) i32 = {
+	match (x) {
+	case let s: str => {
+		if (s.len != 5) { return 1; };
+		if (s[4] != 111u8) { return 2; };
+		return 0;
+	};
+	case let v: i32 => { return 3; };
+	};
+	return 4;
+};
+
+fn main() i32 = {
+	return f(gs);
+};