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/idxfield_compound_slice/case.ww

@@ -1,4 +1,4 @@
-//ww:error "arr[i].field compound on slice field"
+//ww:error "arithmetic on non-numeric type"
 // Lifted from test/wcc/949_idxfield_compound_run.c reject row "fld_he_slice"
 // (#33/rule-7): a compound assign on an indexed-element SLICE field is unwired
 // in both stages and MUST loud-fail. The substring is the shared diagnostic