feat: restructure repo, borrow checker, expanded stdlib

- Reorganize repository to Rust-style layout:
  compiler/bootstrap/  compiler/selfhost/  compiler/tests/
  library/std/  library/runtime/  tests/  tools/
- Add buxs/ Windows-compatible project root
- Add borrow checker tests and implement:
  - Alias analysis (double mutable borrow detection)
  - Use-after-move detection for own T
- Expand standard library:
  - Std::Os: Args, Env, Cwd, Chdir
  - Std::Time: NowMs, NowUs, SleepMs
  - Std::Process: Run, Output
  - Std::Io: PrintInt64 (fixes 32-bit truncation bug)
- Add examples: os_time.bux, process.bux
- Fix PrintInt to use int64_t in C runtime

library/std/Channel.bux

@@ -0,0 +1,35 @@
+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);
+}
+
+func Channel_Recv<T>(ch: *Channel<T>) -> T {
+    var result: T;
+    bux_channel_recv(ch.handle, &result);
+    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);
+}
+
+}