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/r830_slice_base_u16/case.ww

@@ -0,0 +1,8 @@
+//ww:run-exit 30
+// Migrated from 830_cast_base_index row slc_idx.
+package main;
+export fn main() i32 = {
+	let a: [4]u16 = [10u16, 20u16, 30u16, 40u16];
+	let i: int = 2;
+	return a[0:4][i]: i32;
+};