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

lib/Channel.bux

@@ -0,0 +1,56 @@
+module Std::Channel {
+
+extern func bux_channel_new(capacity: int64, elem_size: int64) -> *void;
+extern func bux_channel_send(handle: *void, elem: *void);
+extern func bux_channel_recv(handle: *void, out: *void) -> int;
+extern func bux_channel_close(handle: *void);
+extern func bux_channel_free(handle: *void);
+
+struct Channel<T> {
+    handle: *void;
+}
+
+func Channel_New<T>(capacity: int64) -> Channel<T> {
+    return Channel<T> { handle: bux_channel_new(capacity, sizeof(T)) };
+}
+
+func Channel_Send<T>(ch: *Channel<T>, value: T) {
+    bux_channel_send(ch.handle, (&value) as *void);
+}
+
+func Channel_Recv<T>(ch: *Channel<T>) -> T {
+    var result: T;
+    bux_channel_recv(ch.handle, (&result) as *void);
+    return result;
+}
+
+func Channel_Close<T>(ch: *Channel<T>) {
+    bux_channel_close(ch.handle);
+}
+
+func Channel_Free<T>(ch: *Channel<T>) {
+    bux_channel_free(ch.handle);
+}
+
+/* Convenience wrappers for common types */
+func Channel_SendInt(ch: *Channel<int>, value: int) {
+    bux_channel_send(ch.handle, (&value) as *void);
+}
+
+func Channel_RecvInt(ch: *Channel<int>) -> int {
+    var result: int = 0;
+    bux_channel_recv(ch.handle, (&result) as *void);
+    return result;
+}
+
+func Channel_SendFloat64(ch: *Channel<float64>, value: float64) {
+    bux_channel_send(ch.handle, (&value) as *void);
+}
+
+func Channel_RecvFloat64(ch: *Channel<float64>) -> float64 {
+    var result: float64 = 0.0;
+    bux_channel_recv(ch.handle, (&result) as *void);
+    return result;
+}
+
+}