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/r595_indirect_call_str_arg/case.ww

@@ -0,0 +1,22 @@
+//ww:run
+// #59.5: a str returned by an INDIRECT fn-pointer call, passed
+// directly as a call arg. The pre-fix wwstage nodeisstr/nodeisslice
+// N_CALL arms were callee-name-keyed and fell to false for the
+// N_UN(STAR) callee, so only the ptr word was pushed and the callee
+// read a shifted register file (len=b.ptr).
+package main;
+
+fn hello() str = {
+	return "hello";
+};
+
+fn taker(s: str) i32 = {
+	if (s.len != 5) { return 1; };
+	if (s[0] != 104u8) { return 2; };
+	return 0;
+};
+
+fn main() i32 = {
+	let f = (&hello): *fn() str;
+	return taker((*f)());
+};