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/r744_letcopy_slice/case.ww

@@ -0,0 +1,14 @@
+//ww:run
+package main;
+import os;
+type wsl = struct { a: i32, b: []u8 };
+fn produce() i32 = {
+	let raw: [3]u8 = [1: u8, 2: u8, 3: u8];
+	let p1: wsl;
+	p1.a = 9; p1.b = raw[0:3];
+	let p2: wsl = p1;
+	if (p2.a != 9) { return 11; };
+	if (p2.b.len != 3) { return 12; };
+	return 0;
+};
+export fn main() i32 = { os.exit(produce()); };