test/wcc/data: grow the declarative compiler corpus to 1,224 fixtures

The r-prefixed waves absorb the runtime, reject, and byte-compare rows
of the migrated native carriers; each fixture is one directory with one
case.ww and a //ww:error, //ww:compile, //ww:run, or //ww:run-exit
directive covering both frontends.

test/wcc/data/r771_branched_callee_widen/case.ww

@@ -0,0 +1,14 @@
+//ww:run-exit 72
+// Migrated from 771_widen_transitive row branched_callee_widen.
+package main;
+type underread = !i32;
+type nomem = !void;
+fn pick(b: bool) (size | underread | nomem) = {
+    if (b) { let u: underread; return u; };
+    return 42: size;
+};
+export fn main() i32 = {
+    let r: (size | underread | nomem) = pick(true);
+    if (r is underread) { return 72; };
+    return 99;
+};