wcc: modulo is integer-only; compound ops carry their operand class

Hare's rule (harec check.c binarithm): % and the bitwise/shift five
are integer-only; + - * / need numeric operands. ww grouped % with
the numeric ops, and compound assigns never op-checked at all, so
`a % b` on floats compiled half-lowered (live cs!=ww divergence),
`a %= 2.0` plain-stored the rhs (op silently dropped, both stages),
and `s += "cd"` garbled str headers. Gate both at the checker, both
stages; the cgen float-compound fallbacks and the three unknown-
compound legacy defaults (deref/global/local) demote to rule-7 hard
stops. 34 compound-on-tagged/str/slice fixtures re-pin from the old
cgen "not wired" stops to the earlier checker diagnostics; 3 new
reject fixtures pin the closed shapes.

test/wcc/data/str_pluseq_reject/case.ww

@@ -0,0 +1,9 @@
+//ww:error "arithmetic on non-numeric type"
+// += needs numeric operands; `s += "cd"` passed the op-blind
+// assignability check and cgen garbled the header words silently.
+package main;
+export fn main() i32 = {
+	let s: str = "ab";
+	s += "cd";
+	return 0;
+};