v0.3.0: restructure directories

- src/        ← compiler/selfhost/  (canonical Bux compiler)
- bootstrap/  ← compiler/bootstrap/ (Nim bootstrap)
- lib/        ← library/std/        (standard library)
- rt/         ← library/runtime/    (C runtime)
- tests/      ← compiler/tests/     (unit tests)
- Remove _selfhost/ (built into build/selfhost/ now)
- Update all path references (Makefile, cli.nim, cli.bux, docs)
- Bump version to 0.3.0

tests/testdata/sample.bux

@@ -0,0 +1,11 @@
+import Std::Io::PrintLine;
+
+func Add(a: int32, b: int32) -> int32 {
+    return a + b;
+}
+
+func Main() -> int {
+    let result: int32 = Add(10, 20);
+    PrintLine(c8"Hello from Bux!");
+    return 0;
+}