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_slice_widen/case.ww

@@ -0,0 +1,23 @@
+//ww:run
+// A module-global const []u8 widened into a tagged-union call arg —
+// the slice twin of globalconst_str_widen (path's const dot/dotdot
+// shape).
+package main;
+
+const gb: []u8 = ['x', 'y', 'z'];
+
+fn f(x: ([]u8 | i32)) i32 = {
+	match (x) {
+	case let s: []u8 => {
+		if (len(s) != 3) { return 1; };
+		if (s[2] != 122u8) { return 2; };
+		return 0;
+	};
+	case let v: i32 => { return 3; };
+	};
+	return 4;
+};
+
+fn main() i32 = {
+	return f(gb);
+};